Make |Heap::findGCInfo()| threadsafe.

Source/platform/heap/ThreadState.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 486 matching lines @@
     const GCInfo* findGCInfo(Address);
 #endif
 
     void pushWeakObjectPointerCallback(void*, WeakPointerCallback);
     bool popAndInvokeWeakPointerCallback(Visitor*);
 
     void getStats(HeapStats&);
     HeapStats& stats() { return m_stats; }
     HeapStats& statsAfterLastGC() { return m_statsAfterLastGC; }
 
+    static Mutex& threadAttachMutex();
+
 private:
     explicit ThreadState();
     ~ThreadState();
 
     friend class SafePointBarrier;
 
     void enterSafePoint(StackState, void*);
     NO_SANITIZE_ADDRESS void copyStackUntilSafePointScope();
     void clearSafePointScopeMarker()
     {
@@ skipping 79 matching lines @@
 };
 
 template<> class ThreadStateFor<AnyThread> {
 public:
     static ThreadState* state() { return ThreadState::current(); }
 };
 
 }
 
 #endif // ThreadState_h