[Sync] Have MergeDataAndStartSyncing return a SyncMergeResult

This will allow us to track merge statistics for datatypes. For now none of
the datatypes fill the SyncMergeResult with anything other than an error.

BUG=158576
[email protected]

Review URL: https://chromiumcodereview.appspot.com/11365241

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@168446 0039d316-1c4b-4281-b951-d872f2087c98
31 files changed