Mac Overlays: Allow multiple overlays with sandwich strategy

cc/output/overlay_strategy_single_on_top.h

Index: cc/output/overlay_strategy_single_on_top.h
diff --git a/cc/output/overlay_strategy_single_on_top.h b/cc/output/overlay_strategy_single_on_top.h
index a082e9391ac81c9545b7ed98c3bcb6528f1eb2a4..17c763d8896e6201c088952406011b0dce7b345e 100644
--- a/cc/output/overlay_strategy_single_on_top.h
+++ b/cc/output/overlay_strategy_single_on_top.h
@@ -23,12 +23,12 @@ class CC_EXPORT OverlayStrategySingleOnTop
   OverlayStrategySingleOnTop() {}
   ~OverlayStrategySingleOnTop() override;
 
-  bool TryOverlay(OverlayCandidateValidator* capability_checker,
-                  RenderPassList* render_passes_in_draw_order,
-                  OverlayCandidateList* candidate_list,
-                  const OverlayCandidate& candidate,
-                  QuadList::Iterator candidate_iterator,
-                  float device_scale_factor) override;
+  OverlayResult TryOverlay(OverlayCandidateValidator* capability_checker,
+                           RenderPassList* render_passes_in_draw_order,
+                           OverlayCandidateList* candidate_list,
+                           const OverlayCandidate& candidate,
+                           QuadList::Iterator* candidate_iterator,
+                           float device_scale_factor) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(OverlayStrategySingleOnTop);