Read compositor VSync information from platform, when possible

ash/display/display_manager.h

Index: ash/display/display_manager.h
diff --git a/ash/display/display_manager.h b/ash/display/display_manager.h
index 3450a5d823aacc957e4df634a59cfcafabd4f7d2..949b3ff138760b16049ec05c4b87c687d95ff1f8 100644
--- a/ash/display/display_manager.h
+++ b/ash/display/display_manager.h
@@ -174,11 +174,13 @@ class ASH_EXPORT DisplayManager
                                gfx::Display::Rotation rotation,
                                float ui_scale,
                                const gfx::Insets* overscan_insets,
-                               const gfx::Size& resolution_in_pixels);
+                               const gfx::Size& resolution_in_pixels,
+                               float refresh_rate);
 
   // Returns the display's selected resolution.
   bool GetSelectedResolutionForDisplayId(int64 display_id,
-                                         gfx::Size* resolution_out) const;
+                                         gfx::Size* resolution_out,
+                                         float* refresh_rate_out) const;
 
   // Tells if the virtual resolution feature is enabled.
   bool IsDisplayUIScalingEnabled() const;
@@ -341,7 +343,7 @@ private:
   std::map<int64, DisplayInfo> display_info_;
 
   // Selected resolutions in pixels for displays. Key is the displays' ID.
-  std::map<int64, gfx::Size> resolutions_;
+  std::map<int64, Resolution> resolutions_;
 
   // When set to true, the host window's resize event updates
   // the display's size. This is set to true when running on