Reland of TransportController refactoring.

Reland of TransportController refactoring. (patchset #1 id:1 of https://codereview.webrtc.org/1358413003/ )

Reason for revert:
This CL just landed: https://codereview.chromium.org/1323243006/

Which fixes the FYI bots for the original CL, and breaks them for this revert.

Original issue's description:
> Revert of TransportController refactoring. (patchset #6 id:100001 of https://codereview.webrtc.org/1350523003/ )
>
> Reason for revert:
> This CL causes problems with the WebRTC-in-Chromium FYI bots. Presumably it needs to be done in several steps, where removed files are emptied instead of removed in the first step.
>
> Original issue's description:
> > TransportController refactoring.
> >
> > Getting rid of TransportProxy, and in its place adding a
> > TransportController class which will facilitate access to and manage
> > the lifetimes of Transports. These Transports will now be accessed
> > solely from the worker thread, simplifying their implementation.
> >
> > This refactoring also pulls Transport-related code out of BaseSession.
> > Which means that BaseChannels will now rely on the TransportController
> > interface to create channels, rather than BaseSession.
> >
> > Committed: https://crrev.com/47ee2f3b9f33e8938948c482c921d4e13a3acd83
> > Cr-Commit-Position: refs/heads/master@{#10022}
>
> TBR=pthatcher@webrtc.org,deadbeef@webrtc.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/a81a42f584baa0d93a4b93da9632415e8922450c
> Cr-Commit-Position: refs/heads/master@{#10024}

TBR=pthatcher@webrtc.org,torbjorng@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/cbecd358e032021eac11fb13e04ec7f070d4f407
Cr-Commit-Position: refs/heads/master@{#10036}

[Taylor Brandstetter, 2015-09-23 18:37:45]

Created Reland of TransportController refactoring.

[pthatcher1, 2015-09-23 18:47:00]

lgtm

[Taylor Brandstetter, 2015-09-23 18:49:51]

The CQ bit was checked by deadbeef@webrtc.org

[commit-bot: I haz the power, 2015-09-23 18:50:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1361773005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1361773005/1

[commit-bot: I haz the power, 2015-09-23 18:50:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-23 18:50:40]

Patchset 1 (id:??) landed as
https://crrev.com/cbecd358e032021eac11fb13e04ec7f070d4f407
Cr-Commit-Position: refs/heads/master@{#10036}