[Sync] Fix use of ObserverList by multiple threads.

tools/valgrind/tsan/suppressions.txt

 ############################
 # Real races in Chromium
 {
   bug_23433
   ThreadSanitizer:Race
   fun:*logging*SetMinLogLevel*
 }
 
 {
   bug_24419
@@ skipping 99 matching lines @@
 {
   bug_72548
   ThreadSanitizer:Race
   ...
   fun:JSC::Yarr::Interpreter::*Disjunction*
   fun:JSC::Yarr::Interpreter::interpret*
   fun:JSC::Yarr::interpret*
 }
 
 {
-  bug_82279
-  ThreadSanitizer:Race
-  ...
-  fun:ObserverListBase::Iterator::Iterator
-  fun:syncable::BaseTransaction::NotifyTransactionChangingAndEnding
-}
-
-{
   bug_83564
   ThreadSanitizer:Race
   fun:base::Thread::message_loop
   fun:BrowserThread::IsMessageLoopValid
   fun:ThreadWatcherList::StartWatchingAll
 }
 
 ############################
 # Benign races in Chromium
 {
@@ skipping 408 matching lines @@
   tsanbug_62a
   ThreadSanitizer:Race
   fun:__memset_sse2
 }
 
 {
   tsanbug_62b
   ThreadSanitizer:Race
   fun:__GI_memcpy
 }