[Sync] Fix use of ObserverList by multiple threads.

This doesn't solve the underlying problem (see crbug.com/85481) but will
enable re-landing 88151 for crbug.com/85296.

BUG=82279
TEST=


Review URL: http://codereview.chromium.org/7046067

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88483 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed