Suppressed a few data races in BookmarkModelWorker

tools/valgrind/tsan/suppressions.txt

 ############################
 # Chromium
 
 {
   bug_21259
   ThreadSanitizer:Race
   ...
   fun:*tracked_objects*ThreadData*StartTracking*
 }
 
@@ skipping 50 matching lines @@
   fun:*AppCacheUpdateJobTest*
 }
 
 {
   bug_25385
   ThreadSanitizer:Race
   ...
   fun:*base*ThreadD*E*
 }
 
+{
+  bug_25915
+  ThreadSanitizer:Race
+  fun:*browser_sync*BookmarkModelWorker*
+}
+
 ############################
 # Real races in third_party
 {
   bug_23244 (libevent)
   ThreadSanitizer:Race
   fun:event_*
   fun:event_*
 }
 
 {
@@ skipping 148 matching lines @@
   fun:tzset*
 }
 
 # Benign race in thread-safe function
 {
   fun:mkstemp*
   ThreadSanitizer:Race
   ...
   fun:mkstemp*
 }