Suppress data races under AtExitManager until its threading nature is understood by TSan

TBR=glider
BUG=61753
Review URL: http://codereview.chromium.org/4383001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64921 0039d316-1c4b-4281-b951-d872f2087c98

tools/valgrind/tsan/suppressions.txt

@@ -156,6 +156,13 @@
   fun:*AtExitManager*
 }
 
+{
+  bug_61753
+  ThreadSanitizer:Race
+  ...
+  fun:*AtExitManager*
+}
+
 ############################
 # Data races in tests