Use a FakeVideoTrackSource instead of nullptr in all VideoTrack tests.

webrtc/api/test/fakevideotracksource.h

+/*
+ *  Copyright 2016 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 WEBRTC_API_TEST_FAKEVIDEOTRACKSOURCE_H_
+#define WEBRTC_API_TEST_FAKEVIDEOTRACKSOURCE_H_
+
+#include "webrtc/api/mediastreaminterface.h"
+#include "webrtc/api/videotracksource.h"
+#include "webrtc/media/base/fakevideocapturer.h"
+
+namespace webrtc {
+
+class FakeVideoTrackSource : public VideoTrackSource {
+ public:
+  static rtc::scoped_refptr<FakeVideoTrackSource> Create() {
+    return new rtc::RefCountedObject<FakeVideoTrackSource>();
+  }
+
+  cricket::FakeVideoCapturer* fake_video_capturer() {
+    return &fake_video_capturer_;
+  }
+
+ protected:
+  FakeVideoTrackSource()
+      : VideoTrackSource(&fake_video_capturer_,
+                         rtc::Thread::Current(),
+                         false /* remote */) {}
+  virtual ~FakeVideoTrackSource() {}
+
+ private:
+  cricket::FakeVideoCapturer fake_video_capturer_;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_API_TEST_FAKEVIDEOTRACKSOURCE_H_