mash: remove more shell/shelf WmWindow usage.

ash/wm/immersive_context_ash.cc

Index: ash/wm/immersive_context_ash.cc
diff --git a/ash/wm/immersive_context_ash.cc b/ash/wm/immersive_context_ash.cc
index 580d0777b61ca37891c5e3736845610d5cf231b5..a7345ca297f6ffe8e6a6b5efbbae0b2b7db55e78 100644
--- a/ash/wm/immersive_context_ash.cc
+++ b/ash/wm/immersive_context_ash.cc
@@ -6,6 +6,7 @@
 
 #include "ash/shared/immersive_fullscreen_controller.h"
 #include "ash/shelf/wm_shelf.h"
+#include "ash/shell.h"
 #include "ash/shell_port.h"
 #include "ash/wm/window_state.h"
 #include "ash/wm/window_util.h"
@@ -37,7 +38,7 @@ void ImmersiveContextAsh::OnEnteringOrExitingImmersive(
   // Update the window's immersive mode state for the window manager.
   window_state->set_in_immersive_fullscreen(entering);
 
-  for (WmWindow* root_window : ShellPort::Get()->GetAllRootWindows())
+  for (aura::Window* root_window : Shell::GetAllRootWindows())
     WmShelf::ForWindow(root_window)->UpdateVisibilityState();
 }