Revert "viz: Make primary plane transparent"

This reverts commit 65ae686a.

Reason for revert: breaks legacy page flips, since HardwareDisplayPlaneManager::IsCompatible tests with RGBA and fails.

Original change's description:
> 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/976961
> Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
> Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#545478}

TBR=dnicoara@chromium.org,dcastagna@chromium.org,hoegsberg@chromium.org

Change-Id: If2d49fb3d9b1042653f7d4359b699d784c757d14
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 821944, b/74997524
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Reviewed-on: https://chromium-review.googlesource.com/978581Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545513}

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 = false;
+    output_surface_plane.is_opaque = true;
     current_frame()->overlay_list.push_back(output_surface_plane);
   }