Add context to gfx::Screen calls in support of simultaneous desktop+ash

ash/system/tray/tray_bubble_view.cc

Index: ash/system/tray/tray_bubble_view.cc
diff --git a/ash/system/tray/tray_bubble_view.cc b/ash/system/tray/tray_bubble_view.cc
index e40b88da807178c45af2888b2819f5148cbd2266..ce04b3be16f67bd9bff5ce721a3a44e640872865 100644
--- a/ash/system/tray/tray_bubble_view.cc
+++ b/ash/system/tray/tray_bubble_view.cc
@@ -345,7 +345,7 @@ gfx::Rect TrayBubbleView::GetAnchorRect() {
 
   // TODO(jennyz): May need to add left/right alignment in the following code.
   if (rect.IsEmpty()) {
-    rect = gfx::Screen::GetPrimaryDisplay().bounds();
+    rect = Shell::GetAshScreen()->GetPrimaryDisplay().bounds();
     rect = gfx::Rect(
         base::i18n::IsRTL() ? kPaddingFromRightEdgeOfScreenBottomAlignment :
         rect.width() - kPaddingFromRightEdgeOfScreenBottomAlignment,