Use platform's device scale factor for cursor

ash/display/screen_ash.cc

Index: ash/display/screen_ash.cc
diff --git a/ash/display/screen_ash.cc b/ash/display/screen_ash.cc
index d32deb85c4edd19d028acafb612e43be128233db..9d42e49e26724dc8ee03b2daa08fc3eb282b9539 100644
--- a/ash/display/screen_ash.cc
+++ b/ash/display/screen_ash.cc
@@ -234,7 +234,8 @@ gfx::Display ScreenAsh::GetDisplayNearestWindow(gfx::NativeView window) const {
     return GetPrimaryDisplay();
   const RootWindowSettings* rws = GetRootWindowSettings(root_window);
   if (rws->shutdown)
-    return gfx::Display(1);

[oshima, 2014/04/21 19:39:46]

This was wrong. If the root window is gone, this should return primary, which is
consistent with the behavior during shutdown.

+    return GetPrimaryDisplay();
+
   int64 id = rws->display_id;
   // if id is |kInvaildDisplayID|, it's being deleted.
   DCHECK(id != gfx::Display::kInvalidDisplayID);