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 a1cebe6728db568848e29b89a28815804b6fdc33..34760c9f5da6f1cb5d018d4c6620c4616dbd0933 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -167,6 +167,9 @@ class ASH_EXPORT Shell : ash::CursorDelegate {
   // until the another window who has a different root window becomes active.
   static aura::RootWindow* GetActiveRootWindow();
 
+  // Returns the active RootWindow as a NativeView.
+  static gfx::NativeView GetRootNativeView();
+
   // Returns all root windows.
   static RootWindowList GetAllRootWindows();