mus: Add EventObserver to allow passively listening to UI events

components/mus/public/cpp/BUILD.gn

 # 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.
 
 source_set("cpp") {
   sources = [
     "context_provider.h",
-    "event_matcher.h",
+    "event_matcher_util.h",
     "input_event_handler.h",
     "lib/context_provider.cc",
-    "lib/event_matcher.cc",
+    "lib/event_matcher_util.cc",
     "lib/in_flight_change.cc",
     "lib/in_flight_change.h",
     "lib/output_surface.cc",
     "lib/property_type_converters.cc",
     "lib/scoped_window_ptr.cc",
     "lib/window.cc",
     "lib/window_observer.cc",
     "lib/window_private.cc",
     "lib/window_private.h",
     "lib/window_surface.cc",
@@ skipping 42 matching lines @@
   ]
 
   data_deps = [
     "//components/mus",
   ]
 
   if (is_component_build) {
     deps += [ "//mojo/gles2" ]
   }
 }