TrackObserver DCHECK not on main thread, tests updated.

content/renderer/media/webrtc/track_observer.cc

Index: content/renderer/media/webrtc/track_observer.cc
diff --git a/content/renderer/media/webrtc/track_observer.cc b/content/renderer/media/webrtc/track_observer.cc
index c8ef2d3c52f64a6e5565f274eec5ff1cbf4cb252..4e552ac945741ab01eea646ed5a033c99da9dd2d 100644
--- a/content/renderer/media/webrtc/track_observer.cc
+++ b/content/renderer/media/webrtc/track_observer.cc
@@ -6,7 +6,6 @@
 
 #include "base/bind.h"
 #include "base/location.h"
-#include "base/threading/thread_checker.h"
 
 namespace content {
 
@@ -19,6 +18,7 @@ class CONTENT_EXPORT TrackObserver::TrackObserverImpl
       const scoped_refptr<webrtc::MediaStreamTrackInterface>& track)
       : main_thread_(main_thread), track_(track) {
     // We're on the signaling thread.
+    DCHECK(!main_thread_->BelongsToCurrentThread());
     track->RegisterObserver(this);
   }
 
@@ -65,7 +65,7 @@ class CONTENT_EXPORT TrackObserver::TrackObserverImpl
 
   // webrtc::ObserverInterface implementation.
   void OnChanged() override {
-    DCHECK(signaling_thread_.CalledOnValidThread());
+    DCHECK(!main_thread_->BelongsToCurrentThread());
     webrtc::MediaStreamTrackInterface::TrackState state = track_->state();
     main_thread_->PostTask(FROM_HERE,
         base::Bind(&TrackObserverImpl::OnChangedOnMainThread, this, state));
@@ -81,7 +81,6 @@ class CONTENT_EXPORT TrackObserver::TrackObserverImpl
   const scoped_refptr<base::SingleThreadTaskRunner> main_thread_;
   scoped_refptr<webrtc::MediaStreamTrackInterface> track_;
   OnChangedCallback callback_;  // Only touched on the main thread.
-  base::ThreadChecker signaling_thread_;
 };
 
 TrackObserver::TrackObserver(