Support lossy and lossless <canvas>.toDataURL for webp

Source/platform/graphics/ImageBuffer.h

 /*
  * Copyright (C) 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 136 matching lines @@
         DrawnToAfterSnapshot,
     };
     mutable SnapshotState m_snapshotState;
     OwnPtr<ImageBufferSurface> m_surface;
     ImageBufferClient* m_client;
 };
 
 struct ImageDataBuffer {
     ImageDataBuffer(const IntSize& size, unsigned char* data) : m_data(data), m_size(size) { }
     String PLATFORM_EXPORT toDataURL(const String& mimeType, const double* quality) const;
-    bool PLATFORM_EXPORT encodeImage(const String& mimeType, const double* quality, Vector<char>* output) const;
+    bool PLATFORM_EXPORT encodeImage(const String& mimeType, const double* quality, Vector<char>* output, String* outputMimeType) const;
     unsigned char* pixels() const { return m_data; }
     int height() const { return m_size.height(); }
     int width() const { return m_size.width(); }
     unsigned char* m_data;
     IntSize m_size;
 };
 
 } // namespace blink
 
 #endif // ImageBuffer_h