Add documentation for linux sysroot images

Review URL: https://codereview.chromium.org/1761123002

Cr-Commit-Position: refs/heads/master@{#379164}

docs/linux_sysroot.md

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+# Linux sysroot images
+
+The chromium build system for Linux will (by default) use a sysroot image
+rather than building against the libraries installed on the host system.
+This serves several purposes.  Firstly, it ensures that binaries will run on all
+supported linux systems independent of the packages installed on the build
+machine.  Secondly, it makes the build more hermetic, preventing issues that
+arise for variations among developers' systems.
+
+The sysroot consists of a minimal installation of Debian/stable (or old-stable)
+to ensure maximum compatibility.  Pre-built sysroot images are stored in
+Google Cloud Storage and downloaded during `gclient runhooks`
+
+## Installing the sysroot images
+
+Installation of the sysroot is performed by
+`build/linux/sysroot_scripts/install-sysroot.py`.
+
+This script can be run manually but is normally run as part of gclient
+hooks. When run from hooks this script in a no-op on non-linux platforms.
+
+## Rebuilding the sysroot image
+
+The pre-built sysroot images occasionally needs to be rebuilt.  For example,
+when security updates to debian are released, or when a new package is needed by
+the chromium build.
+
+### Rebuilding
+
+To rebuild the images (without any changes) run the following commands:
+
+    $ cd build/linux/sysroot_scripts
+    $ ./sysroot-creator-wheezy.sh BuildSysrootAll
+
+The above command will rebuild the sysroot for all architectures. To build
+just one architecture use `BuildSysroot<arch>`.  Run the script with no
+arguments for a list of possible architectures.  For example:
+
+    $ ./sysroot-creator-wheezy.sh BuildSysrootAmd64
+
+This command on its own should be a no-op and produce an image identical to
+the one on Google Cloud Storage.
+
+### Updating existing package list
+
+To update packages to the latest versions run:
+
+    $ ./sysroot-creator-wheezy.sh UpdatePackageListsAll
+
+This command will update the package lists that are stored alongside the script.
+If no packages have changed then this script will have no effect.
+
+### Adding new packages
+
+To add a new package, edit the `sysroot-creator-wheezy.sh` script and modify
+the `DEBIAN_PACKAGES` list, then run the update step above
+(`UpdatePackageListsAll`).
+
+### Uploading new images
+
+To upload images to Google Cloud Storage run the following command:
+
+    $ ./sysroot-creator-wheezy.sh UploadSysrootAll <SHA1>
+
+Here you should use the SHA1 of the git revision at which the images were
+created.
+
+Uploading new images to Google Clound Storage requires write permission on the
+`chrome-linux-sysroot` bucket.
+
+### Rolling the sysroot version used by chromium
+
+Once new images have been uploaded the `install-sysroot.py` script needs to be
+updated to reference the new versions.  This process is manual and involves
+updating the `REVISION` and `SHA1SUM` variables in the script.