[webnfc] Implement nfc.watch in blink nfc module.

third_party/WebKit/Source/modules/nfc/MessageCallback.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 MessageCallback_h
 #define MessageCallback_h
 
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class NFCMessage;
+class ScriptState;
 
 class MessageCallback : public GarbageCollectedFinalized<MessageCallback> {
  public:
   virtual ~MessageCallback() {}
   DEFINE_INLINE_VIRTUAL_TRACE() {}
   virtual void handleMessage(const NFCMessage&) = 0;
+
+  void setScriptState(ScriptState* scriptState) { m_scriptState = scriptState; }
+  ScriptState* getScriptState() const { return m_scriptState.get(); }
+
+ private:
+  RefPtr<ScriptState> m_scriptState;
 };
 
 }  // namespace blink
 
 #endif  // MessageCallback_h