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

ash/accelerators/accelerator_controller.cc

Index: ash/accelerators/accelerator_controller.cc
diff --git a/ash/accelerators/accelerator_controller.cc b/ash/accelerators/accelerator_controller.cc
index bc742c74cb6f419442a9cbb330a5129f90865cf4..91ceb7e7c2529d55c09e7afae12b522146e8bc5c 100644
--- a/ash/accelerators/accelerator_controller.cc
+++ b/ash/accelerators/accelerator_controller.cc
@@ -123,7 +123,7 @@ void HandleCycleDisplayMode() {
 }
 
 void HandleSwapPrimaryDisplay() {
-  if (gfx::Screen::GetNumDisplays() > 1) {
+  if (Shell::GetAshScreen()->GetNumDisplays() > 1) {
     Shell::GetInstance()->display_controller()->SetPrimaryDisplay(
         ScreenAsh::GetSecondaryDisplay());
   }