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

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_72913
-  ThreadSanitizer:Race
-  ...
-  fun:ObserverListBase*
-  ...
-  fun:sync_api::SyncManager::SyncInternal::*
-}
-
-{
   bug_82279
   ThreadSanitizer:Race
   fun:ObserverListBase::Iterator::Iterator
   fun:syncable::BaseTransaction::NotifyTransactionChangingAndEnding
 }
 
 {
   bug_83564
   ThreadSanitizer:Race
   fun:base::Thread::message_loop
@@ skipping 415 matching lines @@
   tsanbug_62a
   ThreadSanitizer:Race
   fun:__memset_sse2
 }
 
 {
   tsanbug_62b
   ThreadSanitizer:Race
   fun:__GI_memcpy
 }