Refactor RtcDataChannelHandler.

Refactor RtcDataChannelHandler.
This change splits the WebRtc observer functionality of RtcDataChannelHandler
from the WebKit related functionality. The former belongs with libjingle's
signaling thread and the latter with the main thread. As is, these threads
are one and the same, but this will change later this week.

This is a re-land of: https://codereview.chromium.org/643693007

BUG=369796
TBR=perkj

Committed: https://crrev.com/b545339a3c886a00b899d74398f4db9e84ed6385
Cr-Commit-Position: refs/heads/master@{#302044}

[tommi (sloooow) - chröme, 2014-10-29 21:43:45]

Rebase

[tommi (sloooow) - chröme, 2014-10-29 22:59:38]

tommi@chromium.org changed reviewers:
+ perkj@chromium.org

[tommi (sloooow) - chröme, 2014-10-29 22:59:42]

The CQ bit was checked by tommi@chromium.org

[commit-bot: I haz the power, 2014-10-29 23:01:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/689783002/20001

[commit-bot: I haz the power, 2014-10-30 00:07:22]

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

[commit-bot: I haz the power, 2014-10-30 00:07:34]

Try jobs failed on following builders:
  android_dbg_tests_recipe on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tes...)

[tommi (sloooow) - chröme, 2014-10-30 05:54:31]

The CQ bit was checked by tommi@chromium.org

[commit-bot: I haz the power, 2014-10-30 05:55:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/689783002/20001

[commit-bot: I haz the power, 2014-10-30 06:52:30]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-10-30 06:53:01]

Patchset 2 (id:??) landed as
https://crrev.com/b545339a3c886a00b899d74398f4db9e84ed6385
Cr-Commit-Position: refs/heads/master@{#302044}