Fix race at blink::GCInfoAtBase<T>::index

Source/platform/heap/Visitor.h

Index: Source/platform/heap/Visitor.h
diff --git a/Source/platform/heap/Visitor.h b/Source/platform/heap/Visitor.h
index 83b5f85b6d40556413df4d6011ba68f3b99c49ed..fdca774a3e2528c7660bf862efa061031a7abbc9 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -41,6 +41,7 @@
 #include "wtf/Forward.h"
 #include "wtf/HashMap.h"
 #include "wtf/HashTraits.h"
+#include "wtf/MainThread.h"
 #include "wtf/TypeTraits.h"
 #if ENABLE(GC_PROFILING)
 #include "wtf/InstanceCounter.h"
@@ -390,6 +391,7 @@ template<typename T>
 struct TypenameStringTrait {
     static const String& get()
     {
+        ASSERT(isMainThread());

[haraken, 2015/08/12 12:33:41]

It is not guaranteed that get() is called only by the main thread. It is called
by any GCing thread, which is not necessarily the main thread.

Can we use AtomicallyInitializedStaticReference so that non-main threads can
call get() safely?

         DEFINE_STATIC_LOCAL(String, typenameString, (WTF::extractTypeNameFromFunctionName(WTF::extractNameFunction<T>())));
         return typenameString;
     }