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

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

Index: third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
index e54a4fb7fc4bec91d3c9595f280fb2c66c7bbbb6..cfb07c8fd3cc2d3a42a83add6e1a29da703ade3f 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
+++ b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
@@ -5058,9 +5058,16 @@ void WebGLRenderingContextBase::texImageHelperHTMLCanvasElement(
     // 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,