RegisterContextToDispatch can be called from main canvas too

So calling it should be a noop, not an error.

Change-Id: Id92d427c1e5e9f5795881de30f322a4556f6ade1
Reviewed-on: https://chromium-review.googlesource.com/1042958Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555928}

third_party/blink/renderer/core/html/canvas/canvas_rendering_context_host.h

@@ -63,9 +63,7 @@ class CORE_EXPORT CanvasRenderingContextHost : public GarbageCollectedMixin {
 
   bool IsPaintable() const;
 
-  virtual void RegisterContextToDispatch(CanvasRenderingContext*) {
-    NOTIMPLEMENTED();
-  }
+  virtual void RegisterContextToDispatch(CanvasRenderingContext*) {}
 
  protected:
   virtual ~CanvasRenderingContextHost() {}