Mac Overlays: Allow multiple overlays with sandwich strategy.

cc/output/overlay_strategy_underlay.h

Index: cc/output/overlay_strategy_underlay.h
diff --git a/cc/output/overlay_strategy_underlay.h b/cc/output/overlay_strategy_underlay.h
index e3214318f069421379a83500d982ad1689876783..20c210697f5a364f706922739d632d1f26c31672 100644
--- a/cc/output/overlay_strategy_underlay.h
+++ b/cc/output/overlay_strategy_underlay.h
@@ -21,12 +21,12 @@ class CC_EXPORT OverlayStrategyUnderlay : public OverlayStrategyCommonDelegate {
   OverlayStrategyUnderlay() {}
   ~OverlayStrategyUnderlay() 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(OverlayStrategyUnderlay);