Clean up forward declarations in Source/modules

Source/modules/push_messaging/PushManager.h

 // Copyright 2014 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 PushManager_h
 #define PushManager_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "platform/heap/Handle.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
-class ExecutionContext;
 class ScriptPromise;
 class ScriptState;
 
 class PushManager final : public GarbageCollected<PushManager>, public ScriptWrappable {
     DEFINE_WRAPPERTYPEINFO();
 public:
     static PushManager* create()
     {
         return new PushManager();
     }
 
     ScriptPromise registerPushMessaging(ScriptState*, const String& senderId);
 
     void trace(Visitor*) { }
 
 private:
     PushManager();
 };
 
 } // namespace blink
 
 #endif // PushManager_h