mus: Add EventObserver to allow passively listening to UI events

components/mus/ws/BUILD.gn

Index: components/mus/ws/BUILD.gn
diff --git a/components/mus/ws/BUILD.gn b/components/mus/ws/BUILD.gn
index d9326fea4e1b1eec4253c31a56894ca380b126dd..a5eb3eca8084cd54906bd8010cbe6408aee319c5 100644
--- a/components/mus/ws/BUILD.gn
+++ b/components/mus/ws/BUILD.gn
@@ -25,6 +25,8 @@ source_set("lib") {
     "event_dispatcher.cc",
     "event_dispatcher.h",
     "event_dispatcher_delegate.h",
+    "event_matcher.cc",
+    "event_matcher.h",
     "focus_controller.cc",
     "focus_controller.h",
     "focus_controller_delegate.h",
@@ -164,6 +166,7 @@ test("mus_ws_unittests") {
   sources = [
     "display_unittest.cc",
     "event_dispatcher_unittest.cc",
+    "event_matcher_unittest.cc",
     "focus_controller_unittest.cc",
     "server_window_drawn_tracker_unittest.cc",
     "server_window_surface_manager_test_api.cc",