Added UMA statistics for changing the active window via click or touch events.

ash/shelf/shelf_window_watcher.h

Index: ash/shelf/shelf_window_watcher.h
diff --git a/ash/shelf/shelf_window_watcher.h b/ash/shelf/shelf_window_watcher.h
index 28c9b22c7c4b2d7148e594b85863b82d33a087b8..c9ffcbf7db9ec2136d7dff3039027f14b9e23e1f 100644
--- a/ash/shelf/shelf_window_watcher.h
+++ b/ash/shelf/shelf_window_watcher.h
@@ -103,8 +103,10 @@ class ShelfWindowWatcher : public aura::client::ActivationChangeObserver,
   void FinishObservingRemovedWindow(aura::Window* window);
 
   // aura::client::ActivationChangeObserver overrides:
-  void OnWindowActivated(aura::Window* gained_active,
-                         aura::Window* lost_active) override;
+  void OnWindowActivated(
+      aura::client::ActivationChangeObserver::ActivationReason reason,
+      aura::Window* gained_active,
+      aura::Window* lost_active) override;
 
   // aura::WindowObserver overrides:
   void OnWindowAdded(aura::Window* window) override;