Adding fading effect for paint rectangles in HUD.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 91aefdd4fd31731f4ccbd678b48f77b1813844be..cb605148acbbc3714d0ec7731022a93010c0ad04 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -716,9 +716,12 @@ DrawSwapReadbackResult::DrawResult LayerTreeHostImpl::CalculateRenderPasses(
           root_surface->content_rect());
   bool root_surface_has_contributing_layers =
       !root_surface->layer_list().empty();
+  bool hud_wants_to_draw_ = active_tree_->hud_layer() &&
+                            active_tree_->hud_layer()->IsAnimatingHUDContents();
   if (root_surface_has_contributing_layers &&
       root_surface_has_no_visible_damage &&
-      active_tree_->LayersWithCopyOutputRequest().empty()) {
+      active_tree_->LayersWithCopyOutputRequest().empty() &&
+      !hud_wants_to_draw_) {
     TRACE_EVENT0("cc",
                  "LayerTreeHostImpl::CalculateRenderPasses::EmptyDamageRect");
     frame->has_no_damage = true;