Make |Heap::findGCInfo()| threadsafe.

Source/platform/heap/ThreadState.cpp

 /*
  * 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 873 matching lines @@
     ASSERT(!state->isAtSafePoint());
     state->safePoint(HeapPointersOnStack);
 }
 
 ThreadState::AttachedThreadStateSet& ThreadState::attachedThreads()
 {
     DEFINE_STATIC_LOCAL(AttachedThreadStateSet, threads, ());
     return threads;
 }
 
+#if ENABLE(GC_TRACING)
+const GCInfo* ThreadState::findGCInfoFromAllThreads(Address address)
+{
+    bool needLockForIteration = !isAnyThreadInGC();
+    if (needLockForIteration)
+        threadAttachMutex().lock();
+
+    ThreadState::AttachedThreadStateSet& threads = attachedThreads();
+    for (ThreadState::AttachedThreadStateSet::iterator it = threads.begin(), end = threads.end(); it != end; ++it) {
+        if (const GCInfo* gcInfo = (*it)->findGCInfo(address)) {
+            if (needLockForIteration)
+                threadAttachMutex().unlock();
+            return gcInfo;
+        }
+    }
+    if (needLockForIteration)
+        threadAttachMutex().unlock();
+    return 0;
 }
+#endif
+}