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 ef20b0e6302372e940877892eaa3e4adfd92e34a..2b407aef37e2a880266ce5cc64b84bc582e55b4f 100644
--- a/ash/display/multi_display_manager.cc
+++ b/ash/display/multi_display_manager.cc
@@ -194,7 +194,7 @@ 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();