Document how to submit a tryjob with a pending CL in a DEPS'd repo.

Change-Id: I780524c08cecbd6d6511db456ded0e3676cefc1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308132Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792402}

docs/infra/trybot_usage.md

@@ -57,6 +57,36 @@ $ git cl try -B luci.chromium.try \
   # etc
 ```
 
+## Trying Changes in Dependencies
+
+It is also possible to run a Chromium try job with a pending CL in a separate
+repository that is synced via DEPS. Normally DEPS files specify the SHA1
+revision hash of the dependency. But commits that are part of pending CLs are
+not part of the default
+[refspec](https://git-scm.com/book/en/v2/Git-Internals-The-Refspec) fetched when
+gclient checks out the dependency. Instead, you can specify a symbolic reference
+to your change, like `refs/changes/12/2277112/3`. To determine the ref to use,
+click the "Download" button on the dependency CL in Gerrit, which will show it
+as part of several git commands. Then edit the DEPS file in Chromium.
+
+If, for example, you wanted to test a pending V8 CL in Chromium, you would edit
+the DEPS line, which may look like this:
+
+```
+  'v8_revision': '50bc0b22b15da1410a1be6240a25a184d5896908',
+```
+
+And change it to:
+
+```
+  'v8_revision': 'refs/changes/12/2277112/3',
+```
+
+When you run the try job, gclient will sync in your pending CL. Note that if
+your pending CL is based on a revision that is either older or newer than the
+revision specified in DEPS, the tryjob may fail. You can rebase your CL to be on
+top of the same revision specified in the DEPS file to avoid this.
+
 ## Bugs? Feature requests? Questions?
 
 [File a trooper bug.][3]