Disabling display list 2D canvas by default

As a temporary fix for bug 721727, we are disabling display list 2D
canvas by default.  With this change, display list mode is only engaged
when specifying the --force-display-list-2d-canvas command line option.

BUG=721727

TBR=zmo@chromium.org

Change-Id: I26f0caa3b62506c8d2d051bec208a94c1f51341f
Reviewed-on: https://chromium-review.googlesource.com/563519Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485371}

content/test/gpu/gpu_tests/pixel_expectations.py

@@ -65,3 +65,7 @@ class PixelExpectations(GpuTestExpectations):
               ['mac', 'linux', 'win', 'android'], bug=735228)
     self.Flaky('Pixel_OffscreenCanvasTransferAfterStyleResize',
               ['mac', 'linux', 'win', 'android'], bug=735171)
+
+    # TODO(junov): update reference images
+    self.Fail('Pixel_CSSFilterEffects', ['mac'], bug=721727)
+    self.Fail('Pixel_CSSFilterEffects_NoOverlays', ['mac'], bug=721727)

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

@@ -892,7 +892,10 @@ bool HTMLCanvasElement::ShouldUseDisplayList() {
   if (!RuntimeEnabledFeatures::DisplayList2dCanvasEnabled())
     return false;
 
-  return true;
+  // TODO(crbug.com/721727): Due to a rendering regression with display-
+  // list-2d-canvas, this feature is now disabled by default.  For now, the
+  // feature will only be enabled with --force-display-list-2d-canvas.
+  return false;
 }
 
 std::unique_ptr<ImageBufferSurface>