Mac Overlays: Allow multiple overlays with sandwich strategy

cc/output/overlay_strategy_sandwich.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_OUTPUT_OVERLAY_STRATEGY_SANDWICH_H_
 #define CC_OUTPUT_OVERLAY_STRATEGY_SANDWICH_H_
 
 #include "cc/output/overlay_strategy_common.h"
 
 namespace cc {
 
 // The sandwich strategy looks for a video quad without regard to quads above
 // it. The video is "overlaid" on top of the scene, and then the non-empty
 // parts of the scene are "overlaid" on top of the video. This is only valid
 // for overlay contents that are fully opaque.
 class CC_EXPORT OverlayStrategySandwich : public OverlayStrategyCommonDelegate {
  public:
   OverlayStrategySandwich() {}
   ~OverlayStrategySandwich() 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;
+  QuadList::Iterator 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(OverlayStrategySandwich);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_OVERLAY_STRATEGY_SANDWICH_H_