Don't crash when decoding an invalid handle

sky/engine/core/loader/CanvasImageDecoder.h

 // SKY_ENGINE_CORE_PAINTING_CANVASIMAGE_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 SKY_ENGINE_CORE_LOADER_CANVASIMAGELOADER_H_
 #define SKY_ENGINE_CORE_LOADER_CANVASIMAGELOADER_H_
 
+#include "base/memory/weak_ptr.h"
 #include "mojo/common/data_pipe_drainer.h"
 #include "sky/engine/core/loader/ImageDecoderCallback.h"
 #include "sky/engine/platform/SharedBuffer.h"
 #include "sky/engine/tonic/dart_wrappable.h"
 #include "sky/engine/wtf/OwnPtr.h"
 #include "sky/engine/wtf/text/AtomicString.h"
 
 namespace blink {
 
 class CanvasImageDecoder : public mojo::common::DataPipeDrainer::Client,
                            public RefCounted<CanvasImageDecoder>,
                            public DartWrappable {
   DEFINE_WRAPPERTYPEINFO();
  public:
   static PassRefPtr<CanvasImageDecoder> create(mojo::ScopedDataPipeConsumerHandle handle, PassOwnPtr<ImageDecoderCallback> callback);
   virtual ~CanvasImageDecoder();
 
   // mojo::common::DataPipeDrainer::Client
   void OnDataAvailable(const void*, size_t) override;
   void OnDataComplete() override;
 
  private:
   CanvasImageDecoder(mojo::ScopedDataPipeConsumerHandle handle, PassOwnPtr<ImageDecoderCallback> callback);
 
+  void RejectCallback();
+
   OwnPtr<mojo::common::DataPipeDrainer> drainer_;
   RefPtr<SharedBuffer> buffer_;
   OwnPtr<ImageDecoderCallback> callback_;
+
+  base::WeakPtrFactory<CanvasImageDecoder> weak_factory_;
 };
 
 }  // namespace blink
 
 #endif  // SKY_ENGINE_CORE_LOADER_CANVASIMAGELOADER_H_