cc: Don't subtract transparent overlay from damage.

cc/output/overlay_processor.cc

Index: cc/output/overlay_processor.cc
diff --git a/cc/output/overlay_processor.cc b/cc/output/overlay_processor.cc
index c07eecf4270a8789291b207dbf8513cae7ec373c..594111bd7861a16ebddcb936d6bef68e95eacdc1 100644
--- a/cc/output/overlay_processor.cc
+++ b/cc/output/overlay_processor.cc
@@ -160,7 +160,7 @@ void OverlayProcessor::ProcessForOverlays(
   }
 }
 
-// Subtract on-top overlays from the damage rect, unless the overlays use
+// Subtract on-top opaque overlays from the damage rect, unless the overlays use
 // the backbuffer as their content (in which case, add their combined rect
 // back to the damage at the end).
 // Also subtract unoccluded underlays from the damage rect if we know that the
@@ -173,7 +173,7 @@ void OverlayProcessor::UpdateDamageRect(OverlayCandidateList* candidates,
   gfx::Rect output_surface_overlay_damage_rect;
   gfx::Rect this_frame_underlay_rect;
   for (const OverlayCandidate& overlay : *candidates) {
-    if (overlay.plane_z_order > 0) {
+    if (overlay.plane_z_order > 0 && overlay.is_opaque) {
       const gfx::Rect overlay_display_rect =
           ToEnclosedRect(overlay.display_rect);
       overlay_damage_rect_.Union(overlay_display_rect);