Adds capture to the mojo window_manager.

mojo/services/view_manager/public/cpp/view_observer.h

Index: mojo/services/view_manager/public/cpp/view_observer.h
diff --git a/mojo/services/view_manager/public/cpp/view_observer.h b/mojo/services/view_manager/public/cpp/view_observer.h
index b6b1a702d9014bafa7e276466f67a49556fbe522..4f49fbd560a408222da1e909612f122a5479ff05 100644
--- a/mojo/services/view_manager/public/cpp/view_observer.h
+++ b/mojo/services/view_manager/public/cpp/view_observer.h
@@ -56,6 +56,7 @@ class ViewObserver {
                                    const Rect& old_bounds,
                                    const Rect& new_bounds) {}
 
+  virtual void OnCaptureChanged(View* gained_capture, View* lost_capture) {}
   virtual void OnViewFocusChanged(View* gained_focus, View* lost_focus) {}
   virtual void OnViewActivationChanged(View* gained_active, View* lost_active) {
   }