Update documentation for building instrumented libraries.

- `nih-dbus-tool` no longer exists, and is no longer needed. Remove it
  from the list of packages that need to be installed.
- Run `apt upgrade` after `apt update`, because why not.
- Document several additional build dependencies:
  - git so the GN invocation does not fail when trying to exec scripts
    that use git
  - gyp since several instrumented libraries (e.g. NSS) use gyp
  - fakeroot since several instrumented libraries use fakeroot
- Make the command for mounting /dev/shm more copy-pastable.
- Add basic directions for removing chroots.

Bug: 1369167
Change-Id: I89fd7f8bc8e5ec8d621ccdd0a74f4ae1144065be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404471
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279818}

docs/linux/instrumented_libraries.md

@@ -64,7 +64,8 @@ Enter the chroot and install the necessary packages:
 ```shell
 schroot -c focal_amd64 -u root --directory /home/dev/chromium/src
 apt update
-apt install lsb-release sudo python pkg-config libgtk2.0-bin libdrm-dev nih-dbus-tool help2man
+apt upgrade
+apt install lsb-release sudo python pkg-config libgtk2.0-bin libdrm-dev help2man git fakeroot gyp
 ```
 
 Install library packages:
@@ -79,9 +80,9 @@ exit
 schroot -c focal_amd64 -u `whoami` --directory /home/dev/chromium/src
 ```
 
-On your host, mount `/dev/shm/`.  Replace `<uuid>` with the actual path.
+On your host, mount `/dev/shm/`.  Replace `*` with the actual path if you have multiple chroots.
 ```shell
-sudo mount --bind /dev/shm /run/schroot/mount/focal_amd64-<uuid>/dev/shm
+sudo mount --bind /dev/shm /run/schroot/mount/focal_amd64-*/dev/shm
 ```
 
 Add `depot_tools` to your `PATH`. For example, I have it in `~/dev/depot_tools`,
@@ -127,3 +128,27 @@ the MSAN bot will run on the CQ:
 ```
 CQ_INCLUDE_TRYBOTS=luci.chromium.try:linux_chromium_msan_rel_ng
 ```
+
+## Cleaning up chroots
+
+This can be useful for restarting from scratch with a new chroot, e.g. to
+validate the build instructions above.
+
+```shell
+sudo rm /etc/schroot/chroot.d/focal_amd64.conf
+sudo rm -rf /srv/chroot/focal_amd64
+```
+
+If `rm` complains about active mount points, list the active chroot session(s):
+```shell
+schroot --list --all-sessions
+```
+Which should print something like:
+```
+session:focal_amd64-714a3c01-9dbf-4c98-81c1-90ab8c4c61fe
+```
+
+Then shutdown the chroot session with:
+```shell
+schroot -e -c focal_amd64-714a3c01-9dbf-4c98-81c1-90ab8c4c61fe
+```