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

ash/display/multi_display_manager.cc

Index: ash/display/multi_display_manager.cc
diff --git a/ash/display/multi_display_manager.cc b/ash/display/multi_display_manager.cc
index be5897c90e14893e1c93dace962d68a36e2ca8fa..6f8cdb63c5ac61afe78c5c29a42f23046194d8b8 100644
--- a/ash/display/multi_display_manager.cc
+++ b/ash/display/multi_display_manager.cc
@@ -193,7 +193,7 @@ void MultiDisplayManager::OnNativeDisplaysChanged(
   std::vector<size_t> added_display_indices;
   gfx::Display current_primary;
   if (Shell::HasInstance())
-    current_primary = gfx::Screen::GetPrimaryDisplay();
+    current_primary = Shell::GetAshScreen()->GetPrimaryDisplay();
 
   for (DisplayList::iterator curr_iter = displays_.begin(),
        new_iter = new_displays.begin();