Add documentation on contributing to depended-on projects.

Bug: none
Change-Id: Iacdbac73f92e1c188602a3afe58cc6dd9a4cdf3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5777901
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1345824}

docs/contributing.md

@@ -133,8 +133,10 @@ Chromium workflow is not the same as the GitHub pull request workflow.
 
 ## Uploading a change for review
 
-Note: go through the [commit checklist][commit-checklist] for Chromium before
-uploading a change for review.
+Note: If your change is to a dependent project, see the documentation on
+[changing dependencies](dependencies.md#changing-dependencies). Otherwise, go
+through the [commit checklist][commit-checklist] for Chromium before uploading a
+change for review.
 
 Chromium uses a Gerrit instance hosted at
 <https://chromium-review.googlesource.com> for code reviews. In order to upload

docs/dependencies.md

@@ -1,26 +1,27 @@
 # Managing Chromium dependencies
 
-Chromium uses gclient (part of depot_tools) to manage dependencies. Information
-is stored in DEPS file located in the root of the project. In addition to DEPS
-file, gclient may read git submodules (see
+Chromium uses `gclient` (part of depot_tools) to manage dependencies (e.g. V8,
+WebRTC). Information such as URLs and hashes is stored in the `DEPS` file
+located in the root of the project. In addition to `DEPS`, `gclient` may read
+git submodules (see
 [depot_tools submodules support](https://docs.google.com/document/d/1N_fseFNOj10ETZG3pZ-I30R__w96rYNtvx5y_jFGJWw/view)).
 
-gclient supports three dependency types: git, [gcs](gcs_dependencies.md), and
+`gclient` supports three dependency types: git, [gcs](gcs_dependencies.md), and
 [cipd](cipd_and_3pp.md).
 
 [TOC]
 
 ## Adding dependencies
 
-Add your entry in DEPS file. Then, run `gclient gitmodules` to generate git
-submodules (it will contain .gitmodule change, and gitlink). Edit OWNERS file
-and add gitlink path. Then, run `git add DEPS OWNERS` to stage those files for
-commit, followed by `git commit`. Your change is now ready to be sent for a
-review using `git cl upload`.
+Add your entry in `DEPS`. Then run `gclient gitmodules` to generate git
+submodules; this will contain the `.gitmodule` change and gitlink. Edit the
+`OWNERS` file and add the gitlink path. Then, run `git add DEPS OWNERS` to stage
+those files for commit, followed by `git commit`. Your change is now ready to be
+sent for a review using `git cl upload`.
 
-For example, if new dependency is "src/foo/bar.git", its gitlink path is
-"foo/bar", and OWNERS entry at the top level is `per-file foo/bar=*`. You can
-confirm that by running `git status`. [Example CL](https://crrev.com/c/4923074).
+For example, if your new dependency is "src/foo/bar.git", its gitlink path is
+"foo/bar", and the top level `OWNERS` entry is `per-file foo/bar=*`. You can
+confirm this by running `git status`. [Example CL](https://crrev.com/c/4923074).
 
 ```
 # manual edit of DEPS and OWNERS file (see changes below).
@@ -110,6 +111,30 @@ remote:   https://chromium-review.googlesource.com/c/chromium/src/+/4923074 [DEP
 -- snip --
 ```
 
+## Making changes to dependencies {#changing-dependencies}
+
+If you need a change in a dependency, the general process is to first contribute
+the change upstream, then [roll into Chromium](#rolling-dependencies). Some
+projects (e.g. Skia) are autorolled, but it is good practice to manually roll
+after an upstream change to ensure your change can be successfully rolled and
+there are no resulting compile or test failures.
+
+Upstream projects have a variety of contribution workflows. The two most common
+are Gerrit-based reviews using `git cl upload` (like Chromium itself) and GitHub
+PRs. Some projects have a `CONTRIBUTING.md` file in their root that gives
+instructions.
+
+In most cases, creating a standalone checkout/clone of the project you're
+modifying, outside your Chromium checkout, is the best way to ensure you're
+contributing to upstream `HEAD` and can run the project's presubmit checks.
+Follow the project's contribution instructions (e.g. running `fetch` or
+`gclient sync` as needed, possibly after downloading or cloning the source). If
+you do attempt to create and upload changes directly inside submodules in your
+Chromium checkout, be careful not to commit the new submodule hashes to any
+Chromium changes. You may also need to
+[create symlinks to enable other projects' presubmits](#presubmit-symlinks), or
+else skip them by uploading with `--bypass-hooks`.
+
 ## Rolling dependencies
 
 ### Using gclient
@@ -225,3 +250,20 @@ gclient gitmodules
 The script will create a new .gitmodules files and update all gitlinks. Please
 note that old gitlinks won't be deleted, and you will need to remove them
 manually (see section above for deleting dependencies).
+
+## Appendix: Symlinks to enable other projects' presubmits {#presubmit-symlinks}
+
+Creating the following symlinks (POSIX: `ln -s DEST SRC`, Windows:
+`mklink /D SRC DEST` from an Admin `cmd` prompt) will get other projects'
+presubmit checks working, if you want to upload directly from inside your
+Chromium checkout and don't want to use `--bypass-hooks`. All directories assume
+you are in your Chromium `src` dir. This list is non-exhaustive; please add to
+it as necessary.
+
+* **V8:**
+  * Link `v8/buildtools` to `buildtools`
+  * Link `v8/third_party/depot_tools` to `depot_tools`
+* **WebRTC:**
+  * Link `third_party/webrtc/build` to `build`
+  * Link `third_party/webrtc/buildtools` to `buildtools`
+  * Link `third_party/src` to `.`

third_party/.gitignore

@@ -146,6 +146,7 @@
 /spirv-headers/src
 /spirv-tools/src
 /sqlite4java/lib/
+/src
 /subresource-filter-ruleset/data/*
 /swift-format
 /swift-toolchain