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

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index a7365d822e4ca8c54089ac76f22e475f1576ca0a..20568f507222c3c0815e5afd6b6e6ecf4021ca9a 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -20,6 +20,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "ui/gfx/insets.h"
+#include "ui/gfx/screen.h"
 #include "ui/gfx/size.h"
 
 class CommandLine;
@@ -170,6 +171,9 @@ class ASH_EXPORT Shell : CursorDelegate,
   // until the another window who has a different root window becomes active.
   static aura::RootWindow* GetActiveRootWindow();
 
+  // Returns the global Screen object that's always active in ash.
+  static gfx::Screen* GetScreen();
+
   // Returns all root windows.
   static RootWindowList GetAllRootWindows();