Add Needed Forward Declaration

cc/blink/web_external_texture_layer_impl.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 CC_BLINK_WEB_EXTERNAL_TEXTURE_LAYER_IMPL_H_
 #define CC_BLINK_WEB_EXTERNAL_TEXTURE_LAYER_IMPL_H_
 
 #include "base/bind.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/scoped_vector.h"
 #include "cc/blink/cc_blink_export.h"
 #include "cc/layers/texture_layer_client.h"
 #include "third_party/WebKit/public/platform/WebExternalTextureLayer.h"
 
 namespace blink {
+class WebExternalTextureLayerClient;
 struct WebFloatRect;
 struct WebExternalTextureMailbox;
 }
 
 namespace cc {
 class SingleReleaseCallback;
 class TextureMailbox;
 }
 
 namespace cc_blink {
@@ skipping 37 matching lines @@
   blink::WebExternalTextureLayerClient* client_;
   scoped_ptr<WebLayerImpl> layer_;
   ScopedVector<WebExternalBitmapImpl> free_bitmaps_;
 
   DISALLOW_COPY_AND_ASSIGN(WebExternalTextureLayerImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_WEB_EXTERNAL_TEXTURE_LAYER_IMPL_H_