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Before this CL, all the methods in sync_service.h seemed to be in a
totally arbitrary order. This CL attempts to reorder them to be grouped
in a logical way, and also updates some comments that were out of date.
No functional changes.

Bug: 865936
Change-Id: Ief7c8e8ac6f4eff0e1002dae54df1e96b5e7e00b
Reviewed-on: https://chromium-review.googlesource.com/1146646
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577475}