Add thread check to PacketRouter::SendFeedback, which is called on the pacer thread.

Add thread check to PacketRouter::SendFeedback, which is called on the pacer thread.

Lock is still needed, since other threads may call the methods which
modify the lists.

BUG=None

[nisse-webrtc, 2017-03-31 08:50:43]

The CQ bit was checked by nisse@webrtc.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-31 08:50:47]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.webrtc.org/...

[nisse-webrtc, 2017-03-31 08:53:53]

nisse@webrtc.org changed reviewers:
+ stefan@webrtc.org

[nisse-webrtc, 2017-03-31 08:53:53]

PTAL.

Call thread setup includes

  module_process_thread_->RegisterModule(&receive_side_cc_, RTC_FROM_HERE);

  pacer_thread_->RegisterModule(
      receive_side_cc_.GetRemoteBitrateEstimator(true), RTC_FROM_HERE);

The latter call is the remote_estimator_proxy, which then is added to a
different thread than the estimators for receive side bwe. Which implies that
PacketRouter::SendFeedback is called on the pacer thread.

[commit-bot: I haz the power, 2017-03-31 09:25:27]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-03-31 09:25:28]

Dry run: This issue passed the CQ dry run.

[stefan-webrtc, 2017-03-31 13:53:00]

lgtm

[nisse-webrtc, 2017-03-31 13:59:17]

On 2017/03/31 13:53:00, stefan-webrtc wrote:
> lgtm

Thanks, but already landed as part of the larger cl
https://codereview.webrtc.org/2774623006/ ;-)

I'm closing this one.