viz: DCHECK that clients don't send empty frames

The service rejects a CompostiorFrame if it has an empty render pass
list, or an empty output rect.  Add a DCHECK to ensure that clients
don't send these.

Change-Id: I2a1a25fab8b2e01f5355eaf0378c98d730490863
Reviewed-on: https://chromium-review.googlesource.com/c/1335268Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608087}

services/viz/public/cpp/compositing/compositor_frame_struct_traits.h

@@ -28,6 +28,8 @@ struct StructTraits<viz::mojom::CompositorFrameDataView, viz::CompositorFrame> {
   }
 
   static const viz::RenderPassList& passes(const viz::CompositorFrame& input) {
+    DCHECK(!input_render_pass_list.empty());
+    DCHECK(!input.render_pass_list.back()->output_rect.size().IsEmpty());
     return input.render_pass_list;
   }