Make disabling 2d canvas GPU acceleration for getImageData less aggressive.

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 60 matching lines @@
   // 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,
 
   // GPU readback prevention heuristics
   //====================================
 
-  GetImageDataForcesNoAcceleration = 1,
+  GPUReadbackMinSuccessiveFrames = 3,

[Justin Novosad, 2017/02/16 15:27:20]

This parameter is non-trivial to understand, please add a brief comment to
explain the logic that it controls.

[xiangze.zhang, 2017/02/17 02:13:45]

Done.

+
+  GPUReadbackForcesNoAcceleration = 1,
 
   // When 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 causes the source canvas to
   // switch to non-accelerated when this situation is encountered to
   // prevent future canvas-to-canvas draws from requiring a readback.
   DisableAccelerationToAvoidReadbacks = 0,
 
   // See description of DisableAccelerationToAvoidReadbacks. This is the
   // opposite strategy : accelerate the destination canvas. If both
@@ skipping 110 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