Use RunsTasksInCurrentSequence rather than ThreadChecker in handler.

Update RTCPeerConnectionHandler to check explicitly that the configured
|task_runner_| is the sequence that is is invoked on, rather than using
a separate ThreadChecker.

Bug: 851378
Change-Id: Ib0e4a7f12e12c1e3b92a435244808d177f86cbf2
Reviewed-on: https://chromium-review.googlesource.com/1096277Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566739}

content/renderer/media/webrtc/rtc_peer_connection_handler.h

@@ -18,7 +18,6 @@
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread.h"
-#include "base/threading/thread_checker.h"
 #include "content/common/content_export.h"
 #include "content/renderer/media/webrtc/media_stream_track_metrics.h"
 #include "content/renderer/media/webrtc/rtc_rtp_receiver.h"
@@ -257,8 +256,6 @@ class CONTENT_EXPORT RTCPeerConnectionHandler
   // first call fails.
   bool initialize_called_;
 
-  base::ThreadChecker thread_checker_;
-
   // |client_| is a weak pointer to the blink object (blink::RTCPeerConnection)
   // that owns this object.
   // It is valid for the lifetime of this object.