Added a suppression for http://crbug.com/25645

tools/valgrind/tsan/suppressions.txt

 ############################
 # Chromium
 
 {
   bug_21259
   ThreadSanitizer:Race
   ...
   fun:*tracked_objects*ThreadData*StartTracking*
 }
 
@@ skipping 71 matching lines @@
   fun:*WebCore*WorkerRunLoop*Task*
 }
 
 {
   bug_25385
   ThreadSanitizer:Race
   ...
   fun:*base*ThreadD*E*
 }
 
+{
+  bug_25645
+  ThreadSanitizer:Race
+  fun:*WTF*RefCountedBase*ref*
+  ...
+  fun:*WebCore*StringImpl*
+}
+
 ############################
 # Data races in tests
 
 # TODO(timurrrr): bug item
 {
   Data race on bool in base/thread_unittest
   ThreadSanitizer:Race
   ...
   fun:*ToggleValue*Run*
 }
@@ skipping 117 matching lines @@
   fun:tzset*
 }
 
 # Benign race in thread-safe function
 {
   fun:mkstemp*
   ThreadSanitizer:Race
   ...
   fun:mkstemp*
 }