Do compositing update when switching from software to accelerated canvas

Also, re-enables switching from sw to accelerated canvas.

BUG=704571

Review-Url: https://codereview.chromium.org/2799033002
Cr-Original-Commit-Position: refs/heads/master@{#462706}
Committed: https://chromium.googlesource.com/chromium/src/+/2f0fa56eb8fc2eee27d437e7b6c8038ba5a52d87
Review-Url: https://codereview.chromium.org/2799033002
Cr-Commit-Position: refs/heads/master@{#462900}

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

@@ -1246,6 +1246,7 @@ void HTMLCanvasElement::willDrawImageTo2DContext(CanvasImageSource* source) {
         createAcceleratedImageBufferSurface(opacityMode, &msaaSampleCount);
     if (surface) {
       buffer()->setSurface(std::move(surface));
+      setNeedsCompositingUpdate();
     }
   }
 }

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

@@ -98,7 +98,7 @@ enum {
   // acceleration on the destination first. If that does not succeed,
   // we disable acceleration on the source canvas. Either way, future
   // readbacks are prevented.
-  EnableAccelerationToAvoidReadbacks = 0,
+  EnableAccelerationToAvoidReadbacks = 1,
 
 };  // enum