Support lossy and lossless <canvas>.toDataURL for webp

Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * 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 546 matching lines @@
     double quality;
     double* qualityPtr = nullptr;
     if (!qualityArgument.isEmpty()) {
         v8::Local<v8::Value> v8Value = qualityArgument.v8Value();
         if (v8Value->IsNumber()) {
             quality = v8Value.As<v8::Number>()->Value();
             qualityPtr = &quality;
         }
     }
 
-    String encodingMimeType = toEncodingMimeType(mimeType);
-
     ImageData* imageData = toImageData(BackBuffer);
     ScopedDisposal<ImageData> disposer(imageData);
 
-    // Perform image encoding
     Vector<char> encodedImage;
-    ImageDataBuffer(imageData->size(), imageData->data()->data()).encodeImage(encodingMimeType, qualityPtr, &encodedImage);
-    resultBlob = File::create(encodedImage.data(), encodedImage.size(), encodingMimeType);
+    String encodedMimeType;
+    ImageDataBuffer(imageData->size(), imageData->data()->data()).encodeImage(toEncodingMimeType(mimeType), qualityPtr, &encodedImage, &encodedMimeType);
 
+    resultBlob = File::create(encodedImage.data(), encodedImage.size(), encodedMimeType);
     Platform::current()->mainThread()->postTask(FROM_HERE, bind(&FileCallback::handleEvent, callback, resultBlob));
 }
 
 SecurityOrigin* HTMLCanvasElement::securityOrigin() const
 {
     return document().securityOrigin();
 }
 
 bool HTMLCanvasElement::originClean() const
 {
@@ skipping 356 matching lines @@
 {
     return FloatSize(width(), height());
 }
 
 bool HTMLCanvasElement::isOpaque() const
 {
     return m_context && !m_context->hasAlpha();
 }
 
 } // blink