Refactors maximize mode event handling into its own class

ash/aura/wm_shell_aura.h

Index: ash/aura/wm_shell_aura.h
diff --git a/ash/aura/wm_shell_aura.h b/ash/aura/wm_shell_aura.h
index c26e299a069a3b9713d7edbefe82d22878b442d8..c1084360061100fc0034c9ba22674187d66d01fa 100644
--- a/ash/aura/wm_shell_aura.h
+++ b/ash/aura/wm_shell_aura.h
@@ -50,6 +50,8 @@ class ASH_EXPORT WmShellAura : public WmShell,
   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;