mus: Add EventObserver to allow passively listening to UI events

components/mus/public/interfaces/input_event_matcher.mojom

 // Copyright 2015 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.
 
 module mus.mojom;
 
 import "components/mus/public/interfaces/input_event_constants.mojom";
 import "components/mus/public/interfaces/input_key_codes.mojom";
 import "ui/mojo/geometry/geometry.mojom";
 
@@ skipping 24 matching lines @@
 // types will match this EventMatcher. Similarly, if |key_matcher| is missing,
 // then all key-events will match.
 // An example matcher to match the Ctrl+A accelerator would be:
 // - |type_matcher.type| = mus::mojom::EventType::KEY_PRESSED
 // - |flags_matcher.flags| = mus::mojom::kEventVlagControlDown
 // - |key_matcher.keyboard_code| = mus::mojom::KeyboardCode::A
 //
 // 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;
   // These flags will be stripped from incoming events' flags when comparing
   // against |flags_matcher|.
   EventFlagsMatcher? ignore_flags_matcher;
   KeyEventMatcher? key_matcher;
   PointerKindMatcher? pointer_kind_matcher;
   PointerLocationMatcher? pointer_location_matcher;
 };