viz: Add check to hit_test_region_list in async_layer_tree_frame_sink.

R=riajiang@chromium.org

Change-Id: I1bce7704658f999daf6a7f177d02434661342992
Reviewed-on: https://chromium-review.googlesource.com/c/1349792Reviewed-by: Saman Sami <samans@chromium.org>
Commit-Queue: Alexander Shah <zandershah@google.com>
Cr-Commit-Position: refs/heads/master@{#610656}

cc/mojo_embedder/async_layer_tree_frame_sink.cc

@@ -206,7 +206,7 @@ void AsyncLayerTreeFrameSink::SubmitCompositorFrame(
   else
     hit_test_region_list = client_->BuildHitTestData();
 
-  if (show_hit_test_borders)
+  if (show_hit_test_borders && hit_test_region_list)
     hit_test_region_list->flags |= viz::HitTestRegionFlags::kHitTestDebug;
 
   if (last_submitted_local_surface_id_ != local_surface_id_) {