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

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index 15679c3f2606b46f10593098a35c18a41d17c9bd..2508d9c6ef2a5bba3de9437a328454d451c6c5e3 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -248,7 +248,7 @@ bool SystemTray::IsMouseInNotificationBubble() const {
   if (!notification_bubble_.get())
     return false;
   return notification_bubble_->bubble_view()->GetBoundsInScreen().Contains(
-      gfx::Screen::GetCursorScreenPoint());
+      Shell::GetAshScreen()->GetCursorScreenPoint());
 }
 
 bool SystemTray::CloseBubbleForTest() const {