Converts ShellObserver from aura::Window to ash::WmWindow

ash/wm/workspace/workspace_layout_manager_unittest.cc

Index: ash/wm/workspace/workspace_layout_manager_unittest.cc
diff --git a/ash/wm/workspace/workspace_layout_manager_unittest.cc b/ash/wm/workspace/workspace_layout_manager_unittest.cc
index f3080ec7c229d98804edbb2b6b71e29469595ed6..be39734bda6dca559a098d11bb415b93da728cd5 100644
--- a/ash/wm/workspace/workspace_layout_manager_unittest.cc
+++ b/ash/wm/workspace/workspace_layout_manager_unittest.cc
@@ -74,7 +74,7 @@ class TestShellObserver : public ShellObserver {
   }
 
   void OnFullscreenStateChanged(bool is_fullscreen,
-                                aura::Window* root_window) override {
+                                WmWindow* root_window) override {
     call_count_++;
     is_fullscreen_ = is_fullscreen;
   }