Re-enable overlays for 2d canvas

In this refactor CL:
https://crrev.com/4f6bce25c2f94e59611343ddab1fffcc13f79f07
overlays got disabled by accident, causing performance regressions
on mac.

BUG=792952

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I8acdd0f9157ab4fb31c5cfe62366387cdae7a4ca
Reviewed-on: https://chromium-review.googlesource.com/825702Reviewed-by: Olivia Lai <xlai@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523917}

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

@@ -120,7 +120,7 @@ class PLATFORM_EXPORT CanvasResource_GpuMemoryBuffer final
  private:
   void TearDown();
   GLenum TextureTarget() const final;
-  bool IsOverlayCandidate() const final { return false; }
+  bool IsOverlayCandidate() const final { return true; }
 
   CanvasResource_GpuMemoryBuffer(
       const IntSize&,