Propagate MediaStreamSource state to video tracks the same way as audio.
Also removes unused track states kLive and kFailed.
Since this also required a Video source to exist in all unit tests that create a track, a FakeVideoTrackSource is added and used in tests.

BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1790633002

Cr-Commit-Position: refs/heads/master@{#12098}
diff --git a/webrtc/api/videotrack.cc b/webrtc/api/videotrack.cc
index 54c3cce..bd38025 100644
--- a/webrtc/api/videotrack.cc
+++ b/webrtc/api/videotrack.cc
@@ -20,9 +20,11 @@
                        VideoTrackSourceInterface* video_source)
     : MediaStreamTrack<VideoTrackInterface>(label),
       video_source_(video_source) {
+  video_source_->RegisterObserver(this);
 }
 
 VideoTrack::~VideoTrack() {
+  video_source_->UnregisterObserver(this);
 }
 
 std::string VideoTrack::kind() const {
@@ -56,6 +58,15 @@
   return MediaStreamTrack<VideoTrackInterface>::set_enabled(enable);
 }
 
+void VideoTrack::OnChanged() {
+  RTC_DCHECK(thread_checker_.CalledOnValidThread());
+  if (video_source_->state() == MediaSourceInterface::kEnded) {
+    set_state(kEnded);
+  } else {
+    set_state(kLive);
+  }
+}
+
 rtc::scoped_refptr<VideoTrack> VideoTrack::Create(
     const std::string& id,
     VideoTrackSourceInterface* source) {