InstanceCounter/Oilpan GC tracing: Add clang support

Source/platform/heap/Heap.cpp

Index: Source/platform/heap/Heap.cpp
diff --git a/Source/platform/heap/Heap.cpp b/Source/platform/heap/Heap.cpp
index 6a11b328b0382f56ab08705c96fc94ddba2ac951..4b5d24ac72c5084268cfc9dc091039b8212ff32e 100644
--- a/Source/platform/heap/Heap.cpp
+++ b/Source/platform/heap/Heap.cpp
@@ -1314,6 +1314,7 @@ public:
             return;
         header->mark();
 #if ENABLE(GC_TRACING)
+        MutexLocker locker(objectGraphMutex());
         String className(classOf(objectPointer));
         {
             LiveObjectMap::AddResult result = currentlyLive().add(className, LiveObjectSet());
@@ -1469,6 +1470,12 @@ public:
         objectsToFindPath().add(reinterpret_cast<uintptr_t>(p));
     }
 
+    static Mutex& objectGraphMutex()
+    {
+        AtomicallyInitializedStatic(Mutex&, mutex = *new Mutex);

[tkent, 2014/05/29 12:43:05]

This looks unnatural.
Does the following work?
    AtomicallyInitializedStatic(Mutex, mutex);

+        return mutex;
+    }
+
     static LiveObjectMap& previouslyLive()
     {
         DEFINE_STATIC_LOCAL(LiveObjectMap, map, ());