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The SyncNotificationController is the only consumer of the GSNC, so we
can simplify by merging the two classes together. In addition, remove
displayAndroidMasterSyncDisabledNotification() which had no callers.

The sync notifications do not appear to be tested. I filed
https://crbug.com/679750 to track adding some.

BUG=

Review-Url: https://codereview.chromium.org/2621023002
Cr-Commit-Position: refs/heads/master@{#442992}