Support lossy and lossless <canvas>.toDataURL for webp

Source/platform/graphics/ImageBuffer.h

Index: Source/platform/graphics/ImageBuffer.h
diff --git a/Source/platform/graphics/ImageBuffer.h b/Source/platform/graphics/ImageBuffer.h
index 30ef58220895362c18aa865202fdd2b1d399e1f4..589a16369b4a7e2d794f82a646d6f182cf8621ad 100644
--- a/Source/platform/graphics/ImageBuffer.h
+++ b/Source/platform/graphics/ImageBuffer.h
@@ -154,7 +154,7 @@ private:
 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(); }