Support lossy and lossless <canvas>.toDataURL for webp

Source/core/html/HTMLCanvasElement.cpp

Index: Source/core/html/HTMLCanvasElement.cpp
diff --git a/Source/core/html/HTMLCanvasElement.cpp b/Source/core/html/HTMLCanvasElement.cpp
index 492c0912d8324e6ebe948f49ab2677f6f9dea79e..235a4d9c1d175cb9e60987a11636150ceb70ff37 100644
--- a/Source/core/html/HTMLCanvasElement.cpp
+++ b/Source/core/html/HTMLCanvasElement.cpp
@@ -564,16 +564,14 @@ void HTMLCanvasElement::toBlob(FileCallback* callback, const String& mimeType, c
         }
     }
 
-    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));
 }