Preliminary compositor disabling patch.

cc/output/overlay_strategy_single_on_top.cc

Index: cc/output/overlay_strategy_single_on_top.cc
diff --git a/cc/output/overlay_strategy_single_on_top.cc b/cc/output/overlay_strategy_single_on_top.cc
index 71897ed4596cc1f72bc370bba044136649ff0fe3..566ae1323c42b32e2e34aae68531bfee12167fce 100644
--- a/cc/output/overlay_strategy_single_on_top.cc
+++ b/cc/output/overlay_strategy_single_on_top.cc
@@ -187,6 +187,14 @@ bool OverlayStrategySingleOnTop::Attempt(
   // If the candidate can be handled by an overlay, create a pass for it.
   if (candidates[1].overlay_handled) {
     quad_list.EraseAndInvalidateAllPointers(candidate_iterator);
+    root_render_pass->overlay_rect = gfx::Rect();
+    for (const auto* quad : root_render_pass->quad_list) {
+      if (quad->damaged) {
+        gfx::RectF rect = gfx::RectF(quad->visible_rect);
+        quad->quadTransform().TransformRect(&rect);
+        root_render_pass->overlay_rect.Union(gfx::ToEnclosingRect(rect));
+      }
+    }
     candidate_list->swap(candidates);
     return true;
   }