Lock CrossThreadPersistentRegion until end of weak processing.

third_party/WebKit/Source/platform/heap/PersistentNode.h

Index: third_party/WebKit/Source/platform/heap/PersistentNode.h
diff --git a/third_party/WebKit/Source/platform/heap/PersistentNode.h b/third_party/WebKit/Source/platform/heap/PersistentNode.h
index ebaf0604edc31cd3090c6accc3988d1193c62ec1..41d9c1941fba17863f62749ac8d229597a70373a 100644
--- a/third_party/WebKit/Source/platform/heap/PersistentNode.h
+++ b/third_party/WebKit/Source/platform/heap/PersistentNode.h
@@ -180,15 +180,44 @@ public:
         m_persistentRegion->freePersistentNode(persistentNode);
     }
 
+    class LockScope final {
+        STACK_ALLOCATED();
+    public:
+        LockScope(CrossThreadPersistentRegion& persistentRegion)
+            : m_persistentRegion(persistentRegion)
+        {
+            m_persistentRegion.lock();
+        }
+        ~LockScope()
+        {
+            m_persistentRegion.unlock();
+        }
+    private:
+        CrossThreadPersistentRegion& m_persistentRegion;
+    };
+
     void tracePersistentNodes(Visitor* visitor)
     {
-        MutexLocker lock(m_mutex);
+        // If this assert triggers, you're tracing without being in a LockScope.
+        ASSERT(m_mutex.locked());
         m_persistentRegion->tracePersistentNodes(visitor);
     }
 
     void prepareForThreadStateTermination(ThreadState*);
 
 private:
+    friend class LockScope;
+
+    void lock()
+    {
+        m_mutex.lock();
+    }
+
+    void unlock()
+    {
+        m_mutex.unlock();
+    }
+
     // We don't make CrossThreadPersistentRegion inherit from PersistentRegion
     // because we don't want to virtualize performance-sensitive methods
     // such as PersistentRegion::allocate/freePersistentNode.