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 41bfc6fe98f9e5aa315722db3dd6d03ba3381a83..f35064ed1b37a871645055d4218c07b8e32b68df 100644
--- a/ash/system/tray/tray_bubble_view.cc
+++ b/ash/system/tray/tray_bubble_view.cc
@@ -409,7 +409,7 @@ 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 = ash::Shell::GetAshScreen()->GetPrimaryDisplay().bounds();
     rect = gfx::Rect(
         base::i18n::IsRTL() ? kPaddingFromRightEdgeOfScreenBottomAlignment :
         rect.width() - kPaddingFromRightEdgeOfScreenBottomAlignment,