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

ash/display/display_controller.cc

Index: ash/display/display_controller.cc
diff --git a/ash/display/display_controller.cc b/ash/display/display_controller.cc
index b7361c375631f5872f888d4edf2034aad5febb37..ecc9357c4326c0ac714519816e5f0b158b0559d4 100644
--- a/ash/display/display_controller.cc
+++ b/ash/display/display_controller.cc
@@ -469,10 +469,10 @@ aura::RootWindow* DisplayController::AddRootWindowForDisplay(
 }
 
 void DisplayController::UpdateDisplayBoundsForLayout() {
-  if (gfx::Screen::GetNumDisplays() <= 1)
+  if (ash::Shell::GetAshScreen()->GetNumDisplays() <= 1)
     return;
 
-  DCHECK_EQ(2, gfx::Screen::GetNumDisplays());
+  DCHECK_EQ(2, ash::Shell::GetAshScreen()->GetNumDisplays());
   const gfx::Rect& primary_bounds = GetPrimaryDisplay().bounds();
 
   gfx::Display* secondary_display = GetSecondaryDisplay();