Reland of Adding the ability to create an RtpSender without a track.

Reland of Adding the ability to create an RtpSender without a track. (patchset #1 id:1 of https://codereview.webrtc.org/1426443007/ )

Reason for revert:
Relanding, after changing the expectations of WebRtcBrowserTest.CallAndModifyStream.

Original issue's description:
> Revert of Adding the ability to create an RtpSender without a track. (patchset #8 id:140001 of https://codereview.webrtc.org/1413713003/ )
>
> Reason for revert:
> Causing a compiler warning, and causing WebRtcBrowserTest.CallAndModifyStream to fail.
>
> Original issue's description:
> > Adding the ability to create an RtpSender without a track.
> >
> > This CL also changes AddStream to immediately create a sender, rather
> > than waiting until the track is seen in SDP. And the PeerConnection now
> > builds the list of "send streams" from the list of senders, rather than
> > the collection of local media streams.
> >
> > Committed: https://crrev.com/ac9d92ccbe2b29590c53f702e11dc625820480d5
> > Cr-Commit-Position: refs/heads/master@{#10414}
>
> TBR=pthatcher@webrtc.org,pthatcher@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/8f46c63f6f764254892f4111b54aa1cc8f32eeeb
> Cr-Commit-Position: refs/heads/master@{#10417}

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

Committed: https://crrev.com/6834fa10f142bf5e2275142acb834898911d09ae
Cr-Commit-Position: refs/heads/master@{#10730}

[Taylor Brandstetter, 2015-11-09 22:36:24]

Created Reland of Adding the ability to create an RtpSender without a track.

[pthatcher1, 2015-11-10 20:23:43]

lgtm

[Taylor Brandstetter, 2015-11-20 17:49:34]

The CQ bit was checked by deadbeef@webrtc.org

[Taylor Brandstetter, 2015-11-20 17:49:34]

The patchset sent to the CQ was uploaded after l-g-t-m from pthatcher@webrtc.org
Link to the patchset: https://codereview.webrtc.org/1413983004/#ps300001 (title:
"Fixing merge issue.")

[commit-bot: I haz the power, 2015-11-20 17:49:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1413983004/300001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1413983004/300001

[commit-bot: I haz the power, 2015-11-20 17:50:02]

Committed patchset #3 (id:300001)

[commit-bot: I haz the power, 2015-11-20 17:50:07]

Patchset 3 (id:??) landed as
https://crrev.com/6834fa10f142bf5e2275142acb834898911d09ae
Cr-Commit-Position: refs/heads/master@{#10730}

[Taylor Brandstetter, 2015-11-20 19:42:58]

A revert of this CL (patchset #3 id:300001) has been created in
https://codereview.webrtc.org/1460323002/ by deadbeef@webrtc.org.

The reason for reverting is: Still breaking CallAndModifyStream. Chromium CL
intended to fix it (https://codereview.chromium.org/1435713002/) wasn't
sufficient, because I forgot to call addStream/removeStream on the second
connection..