Converts mash to use Shell

ash/display/window_tree_host_manager.h

Index: ash/display/window_tree_host_manager.h
diff --git a/ash/display/window_tree_host_manager.h b/ash/display/window_tree_host_manager.h
index c145b2cf06bc7db58c3153a2ef8b7e507b5fa29e..df1e96b1095cf85db8a81cee2f909e297763ca3b 100644
--- a/ash/display/window_tree_host_manager.h
+++ b/ash/display/window_tree_host_manager.h
@@ -116,9 +116,6 @@ class ASH_EXPORT WindowTreeHostManager
   // root window host to to new primary display.
   void SetPrimaryDisplayId(int64_t id);
 
-  // Closes all child windows in the all root windows.
-  void CloseChildWindows();
-
   // Returns all root windows. In non extended desktop mode, this
   // returns the primary root window only.
   aura::Window::Windows GetAllRootWindows();