Renamed ChannelObserver to better describe its purpose

ui/ozone/platform/drm/host/gpu_thread_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_GPU_THREAD_OBSERVER_H_
+#define UI_OZONE_PLATFORM_DRM_HOST_GPU_THREAD_OBSERVER_H_
+
+namespace ui {
+
+// Observes the channel state.
+class GpuThreadObserver {
+ public:
+  virtual ~GpuThreadObserver() {}
+
+  // Called when a GPU thread implementation has become available.
+  virtual void OnGpuThreadReady() = 0;
+  // Called when the GPU thread implementation has ceased to be
+  // available.
+  virtual void OnGpuThreadRetired() = 0;
+};
+
+}  // namespace ui
+
+#endif  // UI_OZONE_PLATFORM_DRM_HOST_GPU_THREAD_OBSERVER_H_