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 OffscreenCanvas::SetNeutered() {
  DCHECK(!context_);
  is_neutered_ = true;
  size_.SetWidth(0);
  size_.SetHeight(0);
}

void OffscreenCanvas::SetNeutered() {
  DCHECK(!context_);
  is_neutered_ = true;
  size_.SetWidth(0);
  size_.SetHeight(0);
}
