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The previous logic failed to handle various edge cases, such as when
Sync as a whole is disabled or fails to start up.
The new logic checks whether the user has chosen to sync themes,
independent of the current runtime state of Sync.

Bug: 907027
Change-Id: If9974c0cfd99ad66c28726e319b6f3c565073d5d
Reviewed-on: https://chromium-review.googlesource.com/c/1346391
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611083}