mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/window_tree_client_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/bind.h"
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
@@ skipping 347 matching lines @@
   void OnWindowInputEvent(uint32_t event_id,
                           Id window_id,
                           EventPtr event) override {
     // Ack input events to clear the state on the server. These can be received
     // during test startup. X11Window::DispatchEvent sends a synthetic move
     // event to notify of entry.
     tree()->OnWindowInputEventAck(event_id, mojom::EventResult::HANDLED);
     // Don't log input events as none of the tests care about them and they
     // may come in at random points.
   }
+  void OnEventObserved(EventPtr event) override {}
   void OnWindowSharedPropertyChanged(uint32_t window,
                                      const String& name,
                                      Array<uint8_t> new_data) override {
     tracker_.OnWindowSharedPropertyChanged(window, name, std::move(new_data));
   }
   // TODO(sky): add testing coverage.
   void OnWindowFocused(uint32_t focused_window_id) override {}
   void OnWindowPredefinedCursorChanged(uint32_t window_id,
                                        mojom::Cursor cursor_id) override {
     tracker_.OnWindowPredefinedCursorChanged(window_id, cursor_id);
@@ skipping 1655 matching lines @@
 
 // TODO(sky): make sure coverage of what was
 // WindowManagerTest.SecondEmbedRoot_InitService and
 // WindowManagerTest.MultipleEmbedRootsBeforeWTHReady gets added to window
 // manager
 // tests.
 
 }  // namespace test
 }  // namespace ws
 }  // namespace mus