mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/event_dispatcher_delegate.h

Index: components/mus/ws/event_dispatcher_delegate.h
diff --git a/components/mus/ws/event_dispatcher_delegate.h b/components/mus/ws/event_dispatcher_delegate.h
index a762df51a8ff2c25e20a7d1ae72f21b0eb390f6d..892e90e7960a955917fd19211b709fd7a24397b8 100644
--- a/components/mus/ws/event_dispatcher_delegate.h
+++ b/components/mus/ws/event_dispatcher_delegate.h
@@ -41,6 +41,9 @@ class EventDispatcherDelegate {
                                           const ui::Event& event,
                                           Accelerator* accelerator) = 0;
 
+  // Called when event dispatch could not find a target.

[sky, 2016/04/22 20:18:48]

Clarify that OnAccelerator may still be called.

[James Cook, 2016/04/22 21:25:08]

Done.

+  virtual void OnEventTargetNotFound(const ui::Event& event) = 0;
+
  protected:
   virtual ~EventDispatcherDelegate() {}
 };