Update 'Working in a Chromium repo' documentation.
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 <fd9641b23487c8ee78406c27513624c460600766@google.com>
Commit-Queue: Hal Canary <fd9641b23487c8ee78406c27513624c460600766@google.com>

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