Use a FakeVideoTrackSource instead of nullptr in all VideoTrack tests.

webrtc/api/mediastream_unittest.cc

Index: webrtc/api/mediastream_unittest.cc
diff --git a/webrtc/api/mediastream_unittest.cc b/webrtc/api/mediastream_unittest.cc
index fee92b737a095320642ee3c574d816174021261d..f017466ea227622b0de4a15e9f0f1a9232064e93 100644
--- a/webrtc/api/mediastream_unittest.cc
+++ b/webrtc/api/mediastream_unittest.cc
@@ -13,6 +13,7 @@
 #include "webrtc/api/audiotrack.h"
 #include "webrtc/api/mediastream.h"
 #include "webrtc/api/videotrack.h"
+#include "webrtc/api/test/fakevideotracksource.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "webrtc/base/gunit.h"
@@ -56,7 +57,8 @@ class MediaStreamTest: public testing::Test {
     stream_ = MediaStream::Create(kStreamLabel1);
     ASSERT_TRUE(stream_.get() != NULL);
 
-    video_track_ = VideoTrack::Create(kVideoTrackId, NULL);
+    video_track_ =
+        VideoTrack::Create(kVideoTrackId, FakeVideoTrackSource::Create());
     ASSERT_TRUE(video_track_.get() != NULL);
     EXPECT_EQ(MediaStreamTrackInterface::kInitializing, video_track_->state());