Refactors maximize mode event handling into its own class

ash/mus/bridge/wm_shell_mus.h

Index: ash/mus/bridge/wm_shell_mus.h
diff --git a/ash/mus/bridge/wm_shell_mus.h b/ash/mus/bridge/wm_shell_mus.h
index 5d4fc3f5159e04db60a8386841e839422c97fb15..2be9461237affea0bbb24e6cc1a9c1b26d81bbd8 100644
--- a/ash/mus/bridge/wm_shell_mus.h
+++ b/ash/mus/bridge/wm_shell_mus.h
@@ -64,6 +64,8 @@ class WmShellMus : public WmShell, public ::mus::WindowTreeClientObserver {
   std::unique_ptr<WindowResizer> CreateDragWindowResizer(
       std::unique_ptr<WindowResizer> next_window_resizer,
       wm::WindowState* window_state) override;
+  std::unique_ptr<wm::MaximizeModeEventHandler> CreateMaximizeModeEventHandler()
+      override;
   void OnOverviewModeStarting() override;
   void OnOverviewModeEnded() override;
   AccessibilityDelegate* GetAccessibilityDelegate() override;