sync: annotate call to join SyncCoreThread to keep ThreadRestrictions happy.
sync: annotate call to join SyncCoreThread to keep ThreadRestrictions happy.

Bug 19757 discusses how we block the UI thread on the core thread when shutting
down sync. Until that is fixed, we have to fess up to the Thread police.

BUG=69570,19757
TEST=Start syncing in debug build.  Stop sync.  Don't crash.

Review URL: http://codereview.chromium.org/6328001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@71361 0039d316-1c4b-4281-b951-d872f2087c98

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