Make OffscreenCanvas resizeable

third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.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/offscreencanvas2d/OffscreenCanvasRenderingContext2D.h"
 
 #include "bindings/modules/v8/OffscreenCanvasRenderingContext2DOrWebGLRenderingContextOrWebGL2RenderingContext.h"
 #include "core/frame/ImageBitmap.h"
 #include "core/frame/Settings.h"
 #include "core/workers/WorkerGlobalScope.h"
@@ skipping 82 matching lines @@
 }
 
 int OffscreenCanvasRenderingContext2D::height() const {
   return getOffscreenCanvas()->height();
 }
 
 bool OffscreenCanvasRenderingContext2D::hasImageBuffer() const {
   return !!m_imageBuffer;
 }
 
+void OffscreenCanvasRenderingContext2D::reset() {
+  m_imageBuffer = nullptr;

[xlai (Olivia), 2016/11/21 16:26:57]

This function's implementation doesn't look correct.

Once offscreenCanvas'size is changed, the existing image buffer of its 2d
context is set to nullpointer, means everything that has drawn to 2d context so
far is lost. 

[xlai (Olivia), 2016/11/21 19:56:17]

Ah I just realize that the image buffer (with a new size) is always recreated in
every call to transferToStaticBitmapImage() if it is nullptr. 

+  BaseRenderingContext2D::reset();
+}
+
 ImageBuffer* OffscreenCanvasRenderingContext2D::imageBuffer() const {
   if (!m_imageBuffer) {
     IntSize surfaceSize(width(), height());
     OpacityMode opacityMode = hasAlpha() ? NonOpaque : Opaque;
     std::unique_ptr<ImageBufferSurface> surface;
     if (RuntimeEnabledFeatures::accelerated2dCanvasEnabled()) {
       surface.reset(
           new AcceleratedImageBufferSurface(surfaceSize, opacityMode));
     }
 
@@ skipping 128 matching lines @@
 }
 
 bool OffscreenCanvasRenderingContext2D::isContextLost() const {
   return false;
 }
 
 bool OffscreenCanvasRenderingContext2D::isPaintable() const {
   return this->imageBuffer();
 }
 }