Make |Heap::findGCInfo()| threadsafe.

Source/platform/heap/ThreadState.cpp

Index: Source/platform/heap/ThreadState.cpp
diff --git a/Source/platform/heap/ThreadState.cpp b/Source/platform/heap/ThreadState.cpp
index 87d0c40e847b614df6ebce10370292c8826cf9dc..75a6c81ed865246377001d1202159aba14d2abff 100644
--- a/Source/platform/heap/ThreadState.cpp
+++ b/Source/platform/heap/ThreadState.cpp
@@ -891,4 +891,24 @@ ThreadState::AttachedThreadStateSet& ThreadState::attachedThreads()
     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
 }