Upon clearing, release PersistentNode of cross-thread-persistent also.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PersistentNode_h
 #define PersistentNode_h
 
 #include "platform/PlatformExport.h"
 #include "platform/heap/ThreadState.h"
 #include "wtf/Allocator.h"
@@ skipping 183 matching lines @@
 
     void prepareForThreadStateTermination(ThreadState*);
 
     static bool shouldTracePersistentNode(Visitor*, PersistentNode*);
 
 private:
     // We don't make CrossThreadPersistentRegion inherit from PersistentRegion
     // because we don't want to virtualize performance-sensitive methods
     // such as PersistentRegion::allocate/freePersistentNode.
     OwnPtr<PersistentRegion> m_persistentRegion;
-    Mutex m_mutex;
+
+    // Recursive as prepareForThreadStateTermination() clears a PersistentNode's
+    // associated Persistent<> -- it in turn freeing the PersistentNode. And both
+    // CrossThreadPersistentRegion operations need a lock on the region before
+    // mutating.
+    RecursiveMutex m_mutex;
 };
 
 } // namespace blink
 
 #endif