Renamed ChannelObserver to better describe its purpose

ui/ozone/platform/drm/host/channel_observer.h

-// 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.
-
-#ifndef UI_OZONE_PLATFORM_DRM_HOST_CHANNEL_OBSERVER_H_
-#define UI_OZONE_PLATFORM_DRM_HOST_CHANNEL_OBSERVER_H_
-
-namespace ui {
-
-// Observes the channel state.
-class ChannelObserver {
- public:
-  virtual ~ChannelObserver() {}
-
-  virtual void OnChannelEstablished() = 0;
-  virtual void OnChannelDestroyed() = 0;
-};
-
-}  // namespace ui
-
-#endif  // UI_OZONE_PLATFORM_DRM_HOST_CHANNEL_OBSERVER_H_