Add "alpha" option to PaintWorklet

third_party/WebKit/Source/modules/csspaint/PaintRenderingContext2D.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/csspaint/PaintRenderingContext2D.h"
 
 #include "platform/graphics/ImageBuffer.h"
 #include <memory>
 
 namespace blink {
 
-PaintRenderingContext2D::PaintRenderingContext2D(std::unique_ptr<ImageBuffer> imageBuffer)
-    : m_imageBuffer(std::move(imageBuffer))
+PaintRenderingContext2D::PaintRenderingContext2D(std::unique_ptr<ImageBuffer> imageBuffer, bool hasAlpha)
+    : m_imageBuffer(std::move(imageBuffer)), m_hasAlpha(hasAlpha)
 {
     m_clipAntialiasing = AntiAliased;
     modifiableState().setShouldAntialias(true);
+
+
+    // RecordingImageBufferSurface doesn't call ImageBufferSurface::clear explicitly.
+    DCHECK(m_imageBuffer);
+    m_imageBuffer->canvas()->clear(hasAlpha ? SK_ColorTRANSPARENT : SK_ColorBLACK);
+    m_imageBuffer->didDraw(FloatRect(0, 0, width(), height()));
 }
 
 int PaintRenderingContext2D::width() const
 {
     ASSERT(m_imageBuffer);
     return m_imageBuffer->size().width();
 }
 
 int PaintRenderingContext2D::height() const
 {
@@ skipping 37 matching lines @@
 {
 #if ENABLE(ASSERT)
     SkCanvas* skCanvas = existingDrawingCanvas();
     if (skCanvas) {
         ASSERT(static_cast<size_t>(skCanvas->getSaveCount()) == m_stateStack.size());
     }
 #endif
 }
 
 } // namespace blink