[Sync] Ensure all accesses to SyncInternal's observers are thread safe.

BUG=72913
TEST=unit tests with tsan


Review URL: http://codereview.chromium.org/7044059

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88472 0039d316-1c4b-4281-b951-d872f2087c98

tools/valgrind/tsan/suppressions.txt

@@ -116,18 +116,10 @@
   fun:JSC::Yarr::interpret*
 }
 
-{
-  bug_72913
-  ThreadSanitizer:Race
-  ...
-  fun:ObserverListBase*
-  ...
-  fun:sync_api::SyncManager::SyncInternal::*
-}
-
 {
   bug_82279
   ThreadSanitizer:Race
+  ...
   fun:ObserverListBase::Iterator::Iterator
   fun:syncable::BaseTransaction::NotifyTransactionChangingAndEnding
 }