Work around unknown Mac Intel driver bug causing non-deterministic behavior in GPU readback from 2D

third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 5040 matching lines @@
   } else {
     DCHECK_EQ(depth, 1);
     DCHECK_EQ(unpackImageHeight, 0);
   }
 
   if (functionID == TexImage2D || functionID == TexSubImage2D) {
     // texImageByGPU relies on copyTextureCHROMIUM which doesn't support
     // float/integer/sRGB internal format.
     // TODO(crbug.com/622958): relax the constrains if copyTextureCHROMIUM is
     // upgraded to handle more formats.
-    if (!canvas->renderingContext() ||
-        !canvas->renderingContext()->isAccelerated() ||
-        !canUseTexImageByGPU(functionID, internalformat, type)) {
+    bool forceSoftwareReadbackFrom2DCanvas =
+        this->canvas() && this->canvas()->document().settings() &&
+        this->canvas()
+            ->document()
+            .settings()
+            ->forceSoftwareReadbackFrom2DCanvas();
+    CanvasRenderingContext* sourceContext = canvas->renderingContext();
+    if (!sourceContext || !sourceContext->isAccelerated() ||
+        !canUseTexImageByGPU(functionID, internalformat, type) ||
+        (forceSoftwareReadbackFrom2DCanvas && sourceContext->is2d())) {
       // 2D canvas has only FrontBuffer.
       texImageImpl(functionID, target, level, internalformat, xoffset, yoffset,
                    zoffset, format, type,
                    canvas->copiedImage(FrontBuffer, PreferAcceleration).get(),
                    WebGLImageConversion::HtmlDomCanvas, m_unpackFlipY,
                    m_unpackPremultiplyAlpha, sourceSubRectangle, 1, 0);
       return;
     }
 
     // The GPU-GPU copy path uses the Y-up coordinate system.
@@ skipping 2715 matching lines @@
 
 void WebGLRenderingContextBase::getHTMLOrOffscreenCanvas(
     HTMLCanvasElementOrOffscreenCanvas& result) const {
   if (canvas())
     result.setHTMLCanvasElement(canvas());
   else
     result.setOffscreenCanvas(getOffscreenCanvas());
 }
 
 }  // namespace blink