Clean up CanvasResourceDispatcher on finalizer

We may have pending mojo messages after GC, so we want to drop the
dispatcher as soon as possible.

Bug: 929757,913964
Change-Id: I5789bcbb55aada4a74c67a28758f07686f8911c0
Reviewed-on: https://chromium-review.googlesource.com/c/1489175
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Auto-Submit: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635833}
void HTMLCanvasElement::PushFrame(scoped_refptr<CanvasResource> image,
                                  const SkIRect& damage_rect) {
  NOTIMPLEMENTED();
}

void HTMLCanvasElement::PushFrame(scoped_refptr<CanvasResource> image,
                                  const SkIRect& damage_rect) {
  NOTIMPLEMENTED();
}
