Enable underlay strategy by default in OverlayProcessor

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 6808168e3819cce662ea93deade7a53be80cc579..0f5ec560dd6baeb42bda510ee3ff602826e9382d 100644
--- a/cc/output/overlay_processor.cc
+++ b/cc/output/overlay_processor.cc
@@ -6,6 +6,7 @@
 
 #include "cc/output/output_surface.h"
 #include "cc/output/overlay_strategy_single_on_top.h"
+#include "cc/output/overlay_strategy_underlay.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 #include "ui/gfx/transform.h"
 
@@ -25,6 +26,8 @@ void OverlayProcessor::Initialize() {
   if (candidates) {
     strategies_.push_back(scoped_ptr<Strategy>(
         new OverlayStrategySingleOnTop(candidates, resource_provider_)));
+    strategies_.push_back(scoped_ptr<Strategy>(
+        new OverlayStrategyUnderlay(candidates, resource_provider_)));
   }
 }