Converts overview to use common ash types

ash/wm/common/wm_activation_observer.h

Index: ash/wm/common/wm_activation_observer.h
diff --git a/ash/wm/common/wm_activation_observer.h b/ash/wm/common/wm_activation_observer.h
index ce02022550aa6f7fd9e958b43f94530305644f3b..f5647f80790fc4cfbd3e5c245cb9c813d88bd8e7 100644
--- a/ash/wm/common/wm_activation_observer.h
+++ b/ash/wm/common/wm_activation_observer.h
@@ -16,7 +16,13 @@ class WmWindow;
 class ASH_WM_COMMON_EXPORT WmActivationObserver {
  public:
   virtual void OnWindowActivated(WmWindow* gained_active,
-                                 WmWindow* lost_active) = 0;
+                                 WmWindow* lost_active) {}
+
+  // Called when during window activation the currently active window is
+  // selected for activation. This can happen when a window requested for
+  // activation cannot be activated because a system modal window is active.

[James Cook, 2016/05/27 00:02:51]

nice comment

[sky, 2016/05/27 03:18:03]

This is just a copy of that from ActivationObserver.

+  virtual void OnAttemptToReactivateWindow(WmWindow* request_active,
+                                           WmWindow* actual_active) {}
 
  protected:
   virtual ~WmActivationObserver() {}