Refactors maximize mode event handling into its own class

ash/common/wm_shell.h

Index: ash/common/wm_shell.h
diff --git a/ash/common/wm_shell.h b/ash/common/wm_shell.h
index 6d9498fe2e3dea5760139637be6c61a7f5e89fe4..d9356f348912de4b369d59cf6c8f5539e608354e 100644
--- a/ash/common/wm_shell.h
+++ b/ash/common/wm_shell.h
@@ -38,6 +38,7 @@ class SystemTrayNotifier;
 class WmWindow;
 
 namespace wm {
+class MaximizeModeEventHandler;
 class WindowState;
 }
 
@@ -133,6 +134,9 @@ class ASH_EXPORT WmShell {
       std::unique_ptr<WindowResizer> next_window_resizer,
       wm::WindowState* window_state) = 0;
 
+  virtual std::unique_ptr<wm::MaximizeModeEventHandler>
+  CreateMaximizeModeEventHandler() = 0;
+
   // Called when the overview mode is about to be started (before the windows
   // get re-arranged).
   virtual void OnOverviewModeStarting() = 0;