Disable GPU acceleration on 2D canvas when readbacks are needed

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

Index: third_party/WebKit/Source/platform/graphics/ExpensiveCanvasHeuristicParameters.h
diff --git a/third_party/WebKit/Source/platform/graphics/ExpensiveCanvasHeuristicParameters.h b/third_party/WebKit/Source/platform/graphics/ExpensiveCanvasHeuristicParameters.h
index b1b2d4a20d7c1696bf9f2e342667ee1c23a660a3..f50b5e06a76496184aeb18132b7c4d41ca92df1f 100644
--- a/third_party/WebKit/Source/platform/graphics/ExpensiveCanvasHeuristicParameters.h
+++ b/third_party/WebKit/Source/platform/graphics/ExpensiveCanvasHeuristicParameters.h
@@ -67,6 +67,13 @@ enum {
 
   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