Sync: Move DetermineAccountToUse into new sync_auth_util.h/cc

This is a followup to https://crrev.com/c/1350984.
In particular, this moves the code from /components/browser_sync/ into
/components/sync/, where autofill code can actually access it.

Bug: 903290
Change-Id: Id7b90cf5c1344cb4404d9a672d057a67e74d35b9
Reviewed-on: https://chromium-review.googlesource.com/c/1352160
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611277}
5 files changed