Preliminary compositor disabling patch.

cc/output/direct_renderer.cc

Index: cc/output/direct_renderer.cc
diff --git a/cc/output/direct_renderer.cc b/cc/output/direct_renderer.cc
index 0ae4179deaef141473f276f50eb503cba852f5cb..9937ab18695b3a4b08fdf93e0977c359ea6e9a06 100644
--- a/cc/output/direct_renderer.cc
+++ b/cc/output/direct_renderer.cc
@@ -209,6 +209,13 @@ void DirectRenderer::DrawFrame(RenderPassList* render_passes_in_draw_order,
   DrawingFrame frame;
   frame.render_passes_in_draw_order = render_passes_in_draw_order;
   frame.root_render_pass = root_render_pass;
+
+  overlay_processor_->ProcessForOverlays(render_passes_in_draw_order,
+                                         &frame.overlay_list);
+
+  // Merge quad damage for the root pass.
+  render_passes_in_draw_order->back()->UnionOverlayDamage();
+
   frame.root_damage_rect = Capabilities().using_partial_swap
                                ? root_render_pass->damage_rect
                                : root_render_pass->output_rect;
@@ -218,8 +225,20 @@ void DirectRenderer::DrawFrame(RenderPassList* render_passes_in_draw_order,
   frame.disable_picture_quad_image_filtering =
       disable_picture_quad_image_filtering;
 
-  overlay_processor_->ProcessForOverlays(render_passes_in_draw_order,
-                                         &frame.overlay_list);
+  if (root_render_pass->damage_rect.IsEmpty()) {
+    // If we have a scene that is composed of only overlays then we can
+    // potentially avoid drawing anything. However, if there are any copy
+    // requests we must do the drawing.
+    auto copy_pass = std::find_if(
+        render_passes_in_draw_order->begin(),
+        render_passes_in_draw_order->end(),
+        [](const RenderPass* pass) { return !pass->copy_requests.empty(); });
+    if (copy_pass == render_passes_in_draw_order->end()) {
+      ScheduleOverlays(&frame);
+      render_passes_in_draw_order->clear();
+      return;
+    }
+  }
 
   EnsureBackbuffer();
 
@@ -246,6 +265,7 @@ void DirectRenderer::DrawFrame(RenderPassList* render_passes_in_draw_order,
     }
   }
   FinishDrawingFrame(&frame);
+  ScheduleOverlays(&frame);
 
   render_passes_in_draw_order->clear();
 }