doc: initial version of branch sheriff doc.

Bug: 1034648
Change-Id: I622b68ad3d410b2eb4e528ef705acdc7b5b1d8fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090730Reviewed-by: Eric Foo <efoo@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748869}

docs/branch_sheriff.md

+# Chromium Branch Sheriffing
+
+This document describes how to be a Chromium *branch* sheriff and how sheriffing
+on a branch differs from sheriffing on trunk. For trunk sheriffing guidance, see
+[//docs/sheriff.md][sheriff-md].
+
+[TOC]
+
+## Philosophy
+
+The goals of a branch sheriff are quite similar to those of a trunk sheriff.
+Branch sheriffs need to ensure that:
+
+1. **Compile failures get fixed**, because compile failures on branches block
+all tests (both automated and manual) and consequently reduce our confidence
+in the quality of what we're shipping, possibly to the point of blocking
+releases.
+2. **Consistent test failures get repaired**, because they similarly reduce
+our confidence in the quality of our code.
+
+**Communication** is important for sheriffs in general, but it's particularly
+important for branch sheriffs. Over the course of your shift, you may need to
+coordinate with trunk sheriffs, troopers, release TPMs, and others -- don't
+hesitate to do so, particularly if you have questions.
+
+## Processes
+
+In general, you'll want to follow the same processes outlined in
+[//docs/sheriff.md][sheriff-md]. There are some differences, though.
+
+### Checkout
+
+You'll need to ensure that your checkout is configured to check out the branch
+heads. You can do so by running
+
+```
+  src $ gclient sync --with_branch_heads
+```
+
+> This only needs to be done once, though running it more than once won't hurt.
+
+Once you've done that, you'll be able to check out branches:
+
+```
+  src $ git checkout branch-heads/$BRANCH_NUMBER  # e.g. branch-heads/4044 for M81
+  src $ gclient sync
+```
+
+To determine the appropriate branch number, you can either use
+[chromiumdash][#chromiumdash] or check [milestone.json][milestone-json] directly.
+
+### Flaky tests
+
+You should largely ignore flaky tests for the time being unless you have
+specific reason to believe that a flake was introduced by a cherry-pick to the
+branch in question. If a test is flaky on both trunk *and* a release branch,
+the trunk sheriffs should investigate it.
+
+### Landing changes
+
+When you need to land a change to a branch, you'll need to go through the same
+merge approval process as other cherry-picks. You should feel free to ping the
+relevant release TPM as listed on [chromiumdash][chromiumdash-schedule].
+
+## Tools
+
+### Sheriff-o-Matic
+
+Use the [branch SoM console][sheriff-o-matic] rather than the main chromium console.
+
+### Consoles
+
+Use the [beta][main-beta] and [stable][main-stable] branch consoles rather than the main console.
+
+### Chromiumdash
+
+[chromiumdash][chromiumdash] can help you determine the branch number for a
+particular milestone or channel, along with a host of other useful information:
+
+  * [Branches][chromiumdash-branches] lists the branches for each milestone.
+  * [Releases][chromiumdash-releases] lists the builds currently shipping to
+    each channel, which can help map from channel to milestone or to branch.
+  * [Schedule][chromiumdash-schedule] lists the relevant dates for each
+    milestone and includes the release TPMs responsible for each milestone by
+    platform.
+
+### Rotation
+
+The current branch sheriff is listed [here][rotation-home]. The configuration
+and source of truth for the schedule lives [here][rotation-config]. To swap,
+simply send a CL changing schedule at the bottom of the file.
+
+[chromiumdash]: https://chromiumdash.appspot.com
+[chromiumdash-branches]: https://chromiumdash.appspot.com/branches
+[chromiumdash-releases]: https://chromiumdash.appspot.com/releases
+[chromiumdash-schedule]: https://chromiumdash.appspot.com/schedule
+[main-beta]: https://ci.chromium.org/p/chromium/g/main-beta/console
+[main-stable]: https://ci.chromium.org/p/chromium/g/main-stable/console
+[milestone-json]: https://goto.google.com/chrome-milestone-json
+[rotation-home]: https://goto.google.com/chrome-branch-sheriff-amer-west
+[rotation-config]: https://goto.google.com/chrome-branch-sheriff-amer-west-config
+[sheriff-md]: /docs/sheriff.md
+[sheriff-o-matic]: https://sheriff-o-matic.appspot.com/chrome_browser_release