Support lossy and lossless <canvas>.toDataURL for webp

Source/platform/graphics/ImageBuffer.cpp

Index: Source/platform/graphics/ImageBuffer.cpp
diff --git a/Source/platform/graphics/ImageBuffer.cpp b/Source/platform/graphics/ImageBuffer.cpp
index d1f305d1b878f65dc20522116998343b0293a356..b993e8f422540630c3e784c110cc6d573dbb1c53 100644
--- a/Source/platform/graphics/ImageBuffer.cpp
+++ b/Source/platform/graphics/ImageBuffer.cpp
@@ -341,8 +341,10 @@ void ImageBuffer::putByteArray(Multiply multiplied, const unsigned char* source,
     m_surface->writePixels(info, srcAddr, srcBytesPerRow, destX, destY);
 }
 
-bool ImageDataBuffer::encodeImage(const String& mimeType, const double* quality, Vector<char>* output) const
+bool ImageDataBuffer::encodeImage(const String& mimeType, const double* quality, Vector<char>* output, String* outputMimeType) const
 {
+    ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
+
     Vector<unsigned char>* encodedImage = reinterpret_cast<Vector<unsigned char>*>(output);
 
     if (mimeType == "image/jpeg") {
@@ -351,6 +353,15 @@ bool ImageDataBuffer::encodeImage(const String& mimeType, const double* quality,
             compressionQuality = static_cast<int>(*quality * 100 + 0.5);
         if (!JPEGImageEncoder::encode(*this, compressionQuality, encodedImage))
             return false;
+    } else if (mimeType == "image/webplossless") {
+        int compressionQuality = WEBPImageEncoder::DefaultCompressionQuality;
+        if (quality && *quality >= 0.0 && *quality <= 1.0)
+            compressionQuality = static_cast<int>(*quality * 100 + 0.5);
+        if (!WEBPImageEncoder::encode(*this, compressionQuality, encodedImage, WEBPImageEncoder::Lossless))
+            return false;
+        // webplossless is an encoder selector: map it to the encoded image mimeType: "image/webp".
+        *outputMimeType = "image/webp";
+        return true;
     } else if (mimeType == "image/webp") {
         int compressionQuality = WEBPImageEncoder::DefaultCompressionQuality;
         if (quality && *quality >= 0.0 && *quality <= 1.0)
@@ -363,18 +374,19 @@ bool ImageDataBuffer::encodeImage(const String& mimeType, const double* quality,
         ASSERT(mimeType == "image/png");
     }
 
+    *outputMimeType = mimeType;
     return true;
 }
 
 String ImageDataBuffer::toDataURL(const String& mimeType, const double* quality) const
 {
-    ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
+    String encodedMimeType;
 
     Vector<char> encodedImage;
-    if (!encodeImage(mimeType, quality, &encodedImage))
+    if (!encodeImage(mimeType, quality, &encodedImage, &encodedMimeType))
         return "data:,";
 
-    return "data:" + mimeType + ";base64," + base64Encode(encodedImage);
+    return "data:" + encodedMimeType + ";base64," + base64Encode(encodedImage);
 }
 
 } // namespace blink