Clean up forward declarations in Source/web

Source/web/MediaKeysClientImpl.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 MediaKeysClientImpl_h
 #define MediaKeysClientImpl_h
 
 #include "modules/encryptedmedia/MediaKeysClient.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
 class WebContentDecryptionModule;
-class WebViewImpl;
 
 class MediaKeysClientImpl final : public MediaKeysClient {
 public:
     MediaKeysClientImpl();
 
     // MediaKeysClient implementation.
     virtual PassOwnPtr<WebContentDecryptionModule> createContentDecryptionModule(ExecutionContext*, const String& keySystem) override;
 };
 
 } // namespace blink
 
 #endif // MediaKeysClientImpl_h