Suppress a mac-only leak in AtomicStringTable::create.

tools/valgrind/memcheck/suppressions_mac.txt

 # There are three kinds of suppressions in this file:
 # 1. Third party stuff we have no control over.
 #
 # 2. Intentional unit test errors, stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing.
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system.
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 2376 matching lines @@
 }
 {
   bug_340726
   Memcheck:Leak
   fun:_Znw*
   fun:_ZN4base17LoadNativeLibrary*
   fun:_ZN3gfx26InitializeStaticGLBindings*
   fun:_ZN3gfx9GLSurface30InitializeOneOffImplementation*
   fun:_ZN3gfx9GLSurface24InitializeOneOffForTestsEv
 }
+{
+   bug_340909
+   Memcheck:Leak
+   fun:_Znw*
+   fun:_ZN3WTF17AtomicStringTable6createERNS_13WTFThreadDataE
+   fun:_ZN3WTFL17atomicStringTableEv
+   fun:_ZN3WTFL13atomicStringsEv
+   fun:_ZN3WTFL16addToStringTable*
+   fun:_ZN3WTF12AtomicString18addFromLiteralData*
+   fun:_ZN3WTF12AtomicString*ConstructFromLiteralTagE
+   fun:_ZN3WTF12AtomicString*ConstructFromLiteralTagE
+   fun:_ZN7WebCore9HTMLNames4initEv
+}