mus: Add EventObserver to allow passively listening to UI events

components/mus/public/interfaces/input_event_matcher.mojom

Index: components/mus/public/interfaces/input_event_matcher.mojom
diff --git a/components/mus/public/interfaces/input_event_matcher.mojom b/components/mus/public/interfaces/input_event_matcher.mojom
index f2c89fa4a3e680d003a12771336958d709e581b1..d68e2fb0a73e182e646042520d2d1cc860f5fc0e 100644
--- a/components/mus/public/interfaces/input_event_matcher.mojom
+++ b/components/mus/public/interfaces/input_event_matcher.mojom
@@ -42,6 +42,7 @@ struct EventFlagsMatcher {
 // A matcher to match any key-press event would be:
 // - |type_matcher.type| = mus::mojom::EventType::KEY_PRESSED
 struct EventMatcher {
+  // TODO(jamescook): Move this to somewhere accelerator-specific.
   AcceleratorPhase accelerator_phase;
   EventTypeMatcher? type_matcher;
   EventFlagsMatcher? flags_matcher;