Update 'Working in a Chromium repo' documentation.

No-Try: true
Docs-Preview: https://skia.org/?cl=118521
Change-Id: Ibc639d10440ad449c875fe897af4f941fe4d4397
Reviewed-on: https://skia-review.googlesource.com/118521
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
1 file changed