Mac Overlays: Add sandwich strategy

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 baae69dd10ed2bbe04e9178b1948c743a3356d23..ab334ec95344aef8d5d98a4cf14a50a001430a54 100644
--- a/cc/output/overlay_processor.cc
+++ b/cc/output/overlay_processor.cc
@@ -17,15 +17,10 @@ OverlayProcessor::OverlayProcessor(OutputSurface* surface) : surface_(surface) {
 
 void OverlayProcessor::Initialize() {
   DCHECK(surface_);
-
-  OverlayCandidateValidator* candidates =
+  OverlayCandidateValidator* validator =
       surface_->GetOverlayCandidateValidator();
-  if (candidates) {
-    strategies_.push_back(
-        scoped_ptr<Strategy>(new OverlayStrategySingleOnTop(candidates)));
-    strategies_.push_back(
-        scoped_ptr<Strategy>(new OverlayStrategyUnderlay(candidates)));
-  }
+  if (validator)
+    validator->GetStrategies(&strategies_);
 }
 
 OverlayProcessor::~OverlayProcessor() {}
@@ -35,8 +30,10 @@ void OverlayProcessor::ProcessForOverlays(
     OverlayCandidateList* candidate_list) {
   for (StrategyList::iterator it = strategies_.begin(); it != strategies_.end();
        ++it) {
-    if ((*it)->Attempt(render_passes_in_draw_order, candidate_list))
+    if ((*it)->Attempt(render_passes_in_draw_order, candidate_list,
+                       surface_->device_scale_factor())) {
       return;
+    }
   }
 }