viz: Make primary plane transparent

This CL enables alpha blending at scanout for the primary plane.
In this way chromecast will be able to punch a hole in the primary
plane and show content underneath.

Note that on platform using legacy pageflip we'll still be
scanning out without alpha blending (crrev.com/c/964984).

Bug: 821944, b/74997524
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I654935dc630f7a9f6cb4717cd574a21a2749c442
Reviewed-on: https://chromium-review.googlesource.com/976961Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545478}

components/viz/service/display/direct_renderer.cc

@@ -280,7 +280,7 @@ void DirectRenderer::DrawFrame(RenderPassList* render_passes_in_draw_order,
     output_surface_plane.format = output_surface_->GetOverlayBufferFormat();
     output_surface_plane.use_output_surface_for_resource = true;
     output_surface_plane.overlay_handled = true;
-    output_surface_plane.is_opaque = true;
+    output_surface_plane.is_opaque = false;
     current_frame()->overlay_list.push_back(output_surface_plane);
   }