GitLab

Introduces a GetMutableEntityForSyncId helper, the code for which was
duplicated many times before, and updates a few comments.

Bug: none
Change-Id: I775a5912990be1d80d7476ff2af8177b05928f14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1866633Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707302}