Disable GPU acceleration on 2D canvas when readbacks are needed

third_party/WebKit/Source/platform/graphics/ExpensiveCanvasHeuristicParameters.h

 // Copyright 2015 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.
 
 #ifndef ExpensiveCanvasHeuristicParameters_h
 #define ExpensiveCanvasHeuristicParameters_h
 
 namespace blink {
 
 namespace ExpensiveCanvasHeuristicParameters {
@@ skipping 49 matching lines @@
   // lists. Rationale: The allocation of large textures for canvas
   // tends to starve the compositor, and increase the probability of
   // failure of subsequent allocations required for double buffering.
   PreferDisplayListOverGpuSizeThreshold = 4096 * 4096,
 
   // Disable Acceleration heuristic parameters
   //===========================================
 
   GetImageDataForcesNoAcceleration = 1,
 
+  // Whan a canvas is used as a source image, if its destination is
+  // non-accelerated and the source canvas is accelerated, a readback
+  // from the gpu is necessary. This option cause the source canvas to

[Stephen White, 2016/10/04 18:40:14]

Nit: cause -> causes

+  // switch to non-accelerated when this situation is encountered to
+  // prevent future canvas to canvas draws from requiring a readback.

[Stephen White, 2016/10/04 18:40:14]

uNit: "canvas to canvas" -> "canvas-to-canvas"

+  DisableAccelerationToAvoidReadbacks = 1,
+
   // When drawing very large images to canvases, there is a point where
   // GPU acceleration becomes inefficient due to texture upload overhead,
   // especially when the image is large enough that it is likely to
   // monopolize the texture cache, and when it is being downsized to the
   // point that few of the upload texels are actually sampled. When both
   // of these conditions are met, we disable acceleration.
   DrawImageTextureUploadSoftSizeLimit = 4096 * 4096,
   DrawImageTextureUploadSoftSizeLimitScaleThreshold = 4,
   DrawImageTextureUploadHardSizeLimit = 8192 * 8192,
 
@@ skipping 96 matching lines @@
 // before the rendering pipeline may be switched. Having this set
 // to more than 1 increases the sample size of usage data before a
 // decision is made, improving the accuracy of heuristics.
 const int MinFramesBeforeSwitch = 3;
 
 }  // namespace ExpensiveCanvasHeuristicParameters
 
 }  // namespace blink
 
 #endif