Move pc/media_stream_track.h to the api/ directory

This file is being accessed from Chrome. Moving it lessens the
dependency of Chrome on files in the pc/ directory, and allows
easier refactoring of pc/.

Bug: webrtc:11967
Change-Id: Iccd568f84e9cf4086e37c58db1b4cba6c376f413
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187489
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32378}

api/BUILD.gn

@@ -98,6 +98,7 @@ rtc_library("media_stream_interface") {
   sources = [
     "media_stream_interface.cc",
     "media_stream_interface.h",
+    "media_stream_track.h",
     "notifier.h",
   ]
   deps = [

api/media_stream_track.h

@@ -0,0 +1,63 @@
+/*
+ *  Copyright 2011 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef API_MEDIA_STREAM_TRACK_H_
+#define API_MEDIA_STREAM_TRACK_H_
+
+#include <string>
+
+#include "api/media_stream_interface.h"
+#include "api/notifier.h"
+
+namespace webrtc {
+
+// MediaTrack implements the interface common to AudioTrackInterface and
+// VideoTrackInterface.
+template <typename T>
+class MediaStreamTrack : public Notifier<T> {
+ public:
+  typedef typename T::TrackState TypedTrackState;
+
+  std::string id() const override { return id_; }
+  MediaStreamTrackInterface::TrackState state() const override {
+    return state_;
+  }
+  bool enabled() const override { return enabled_; }
+  bool set_enabled(bool enable) override {
+    bool fire_on_change = (enable != enabled_);
+    enabled_ = enable;
+    if (fire_on_change) {
+      Notifier<T>::FireOnChanged();
+    }
+    return fire_on_change;
+  }
+  void set_ended() { set_state(MediaStreamTrackInterface::TrackState::kEnded); }
+
+ protected:
+  explicit MediaStreamTrack(const std::string& id)
+      : enabled_(true), id_(id), state_(MediaStreamTrackInterface::kLive) {}
+
+  bool set_state(MediaStreamTrackInterface::TrackState new_state) {
+    bool fire_on_change = (state_ != new_state);
+    state_ = new_state;
+    if (fire_on_change)
+      Notifier<T>::FireOnChanged();
+    return true;
+  }
+
+ private:
+  bool enabled_;
+  const std::string id_;
+  MediaStreamTrackInterface::TrackState state_;
+};
+
+}  // namespace webrtc
+
+#endif  // API_MEDIA_STREAM_TRACK_H_