Reland of Moving MediaStreamSignaling logic into PeerConnection.

Reland of Moving MediaStreamSignaling logic into PeerConnection. (patchset #1 id:1 of https://codereview.webrtc.org/1403633005/ )

Reason for reland:
The original CL actually didn't break browser_tests; it was
just a coincidence that it started failing.

Original issue's description:
> Revert of Moving MediaStreamSignaling logic into PeerConnection. (patchset #10 id:180001 of https://codereview.webrtc.org/1393563002/ )
>
> Reason for revert:
> Broke browser_tests on Mac. Still need to investigate the cause.
>
> Original issue's description:
> > Moving MediaStreamSignaling logic into PeerConnection.
> >
> > This needs to happen because in the future, m-lines will be offered
> > based on the set of RtpSenders/RtpReceivers, rather than the set of
> > tracks that MediaStreamSignaling knows about.
> >
> > Besides that, MediaStreamSignaling was a "glue class" without
> > a clearly defined role, so it going away is good for other
> > reasons as well.
> >
> > Committed: https://crrev.com/97c392935411398b506861601c82e31d95c591f0
> > Cr-Commit-Position: refs/heads/master@{#10268}
>
> TBR=pthatcher@webrtc.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/fc648b6d934e936f4d9a32c813364b331536ec3b
> Cr-Commit-Position: refs/heads/master@{#10269}

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

Committed: https://crrev.com/ab9b2d1516cad017c6e0236c468934582530c965
Cr-Commit-Position: refs/heads/master@{#10277}

[Taylor Brandstetter, 2015-10-14 00:24:08]

Created Reland of Moving MediaStreamSignaling logic into PeerConnection.

[Taylor Brandstetter, 2015-10-14 18:32:11]

The CQ bit was checked by deadbeef@webrtc.org

[commit-bot: I haz the power, 2015-10-14 18:32:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1404473005/280001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1404473005/280001

[commit-bot: I haz the power, 2015-10-14 18:33:17]

Committed patchset #2 (id:280001)

[commit-bot: I haz the power, 2015-10-14 18:33:33]

Patchset 2 (id:??) landed as
https://crrev.com/ab9b2d1516cad017c6e0236c468934582530c965
Cr-Commit-Position: refs/heads/master@{#10277}