mus: Add EventObserver to allow passively listening to UI events

components/mus/public/cpp/lib/event_matcher.cc

-// 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.
-
-#include "components/mus/public/cpp/event_matcher.h"
-
-namespace mus {
-
-mojom::EventMatcherPtr CreateKeyMatcher(mojom::KeyboardCode code, int flags) {
-  mojom::EventMatcherPtr matcher(mojom::EventMatcher::New());
-  matcher->type_matcher = mojom::EventTypeMatcher::New();
-  matcher->flags_matcher = mojom::EventFlagsMatcher::New();
-  matcher->ignore_flags_matcher = mojom::EventFlagsMatcher::New();
-  // Ignoring these makes most accelerator scenarios more straight forward. Code
-  // that needs to check them can override this setting.
-  matcher->ignore_flags_matcher->flags = mojom::kEventFlagCapsLockOn |
-                                         mojom::kEventFlagScrollLockOn |
-                                         mojom::kEventFlagNumLockOn;
-  matcher->key_matcher = mojom::KeyEventMatcher::New();
-
-  matcher->type_matcher->type = mus::mojom::EventType::KEY_PRESSED;
-  matcher->flags_matcher->flags = flags;
-  matcher->key_matcher->keyboard_code = code;
-  return matcher;
-}
-
-}  // namespace mus