Read compositor VSync information from platform, when possible

ash/display/display_change_observer_chromeos.h

Index: ash/display/display_change_observer_chromeos.h
diff --git a/ash/display/display_change_observer_chromeos.h b/ash/display/display_change_observer_chromeos.h
index c75c00be4dad37f3d36455dfa602a6259ad2f445..bc673f46cd80155ea70dc9c76286eb72311497a8 100644
--- a/ash/display/display_change_observer_chromeos.h
+++ b/ash/display/display_change_observer_chromeos.h
@@ -13,7 +13,7 @@
 namespace ash {
 namespace internal {
 
-struct Resolution;
+struct DisplayMode;
 
 // An object that observes changes in display configuration and
 // update DisplayManagers.
@@ -23,7 +23,7 @@ class DisplayChangeObserver
       public ShellObserver {
  public:
   // Returns the resolution list.
-  ASH_EXPORT static std::vector<Resolution> GetResolutionList(
+  ASH_EXPORT static std::vector<DisplayMode> GetDisplayModeList(
       const chromeos::OutputConfigurator::OutputSnapshot& output);
 
   DisplayChangeObserver();