Add missing null check to OffscreenCanvas::PushFrame

BUG=866273

Change-Id: Ie53ffb789b005089813f11f26f4425a8f8884b0f
Reviewed-on: https://chromium-review.googlesource.com/1147461Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577326}

third_party/blink/renderer/core/offscreencanvas/offscreen_canvas.cc

@@ -388,7 +388,7 @@ void OffscreenCanvas::PushFrame(scoped_refptr<CanvasResource> canvas_resource,
   DCHECK(needs_push_frame_);
   needs_push_frame_ = false;
   current_frame_damage_rect_.join(damage_rect);
-  if (current_frame_damage_rect_.isEmpty())
+  if (current_frame_damage_rect_.isEmpty() || !canvas_resource)
     return;
   base::TimeTicks commit_start_time = WTF::CurrentTimeTicks();
   GetOrCreateResourceDispatcher()->DispatchFrame(std::move(canvas_resource),