[Chromecast] Turn CastContentWindow into an abstract interface.

chromecast/android/cast_metrics_helper_android.h

-// 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.
-
-#ifndef CHROMECAST_ANDROID_CAST_METRICS_HELPER_ANDROID_H_
-#define CHROMECAST_ANDROID_CAST_METRICS_HELPER_ANDROID_H_
-
-#include <jni.h>
-#include <vector>
-
-#include "base/macros.h"
-
-namespace chromecast {
-
-class CastMetricsHelperAndroid {
- public:
-  // Registers the JNI methods for CastMetricsHelperAndroid.
-  static bool RegisterJni(JNIEnv* env);
-
- private:
-  DISALLOW_IMPLICIT_CONSTRUCTORS(CastMetricsHelperAndroid);
-};
-
-}  // namespace chromecast
-
-#endif  // CHROMECAST_ANDROID_CAST_METRICS_HELPER_ANDROID_H_