Introdcue AshWindowTreeHost and move ash/chrome specific code in WTH to 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 7195004dd630761ce48b2a36541b4c93894ada8f..0f8c1c2ee50775fc249436db6a1213b13d9da37a 100644
--- a/ash/accelerators/accelerator_controller.cc
+++ b/ash/accelerators/accelerator_controller.cc
@@ -19,6 +19,7 @@
 #include "ash/focus_cycler.h"
 #include "ash/gpu_support.h"
 #include "ash/ime_control_delegate.h"
+#include "ash/host/ash_window_tree_host.h"
 #include "ash/magnifier/magnification_controller.h"
 #include "ash/magnifier/partial_magnification_controller.h"
 #include "ash/media_delegate.h"
@@ -498,7 +499,7 @@ bool HandleToggleFullscreen(ui::KeyboardCode key_code) {
 }
 
 bool HandleToggleRootWindowFullScreen() {
-  Shell::GetPrimaryRootWindow()->GetHost()->ToggleFullScreen();
+  Shell::GetPrimaryRootWindowController()->ash_host()->ToggleFullScreen();
   return true;
 }