Remove decoder-thread instantiation for senders.

Remove decoder-thread instantiation for senders.

Reduces number of running (high-priority) threads, even though the
thread was practically blocked all the time.

Also adding DCHECKs to make sure we're not trying to use certain
sender-only methods on receivers and vice versa.

BUG=webrtc:1675, webrtc:1695
R=stefan@webrtc.org

Committed: https://crrev.com/cd4a9bd22596c624e5a7f2646a80a40317451882
Cr-Commit-Position: refs/heads/master@{#9534}

[pbos-webrtc, 2015-07-02 12:32:47]

PTAL, yay.

[pbos-webrtc, 2015-07-02 12:32:52]

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

[commit-bot: I haz the power, 2015-07-02 12:33:00]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1222193003/1

[stefan-webrtc, 2015-07-02 12:40:33]

lgtm

[pbos-webrtc, 2015-07-02 12:40:58]

The CQ bit was unchecked by pbos@webrtc.org

[pbos-webrtc, 2015-07-02 12:41:06]

The CQ bit was checked by pbos@webrtc.org

[commit-bot: I haz the power, 2015-07-02 12:41:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1222193003/1

[commit-bot: I haz the power, 2015-07-02 13:32:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-02 13:32:28]

Patchset 1 (id:??) landed as
https://crrev.com/cd4a9bd22596c624e5a7f2646a80a40317451882
Cr-Commit-Position: refs/heads/master@{#9534}

[tommi, 2015-07-06 09:44:28]

tommi@webrtc.org changed reviewers:
+ tommi@webrtc.org

[tommi, 2015-07-06 09:44:29]

Nice change (lgtm!).  I don't suppose we can start adding thread checks for
variables such as sender_, the start/stop methods etc that don't have locks,
etc?

[pbos-webrtc, 2015-07-06 09:55:12]

On 2015/07/06 09:44:29, tommi (webrtc) wrote:
> Nice change (lgtm!).  I don't suppose we can start adding thread checks for
> variables such as sender_, the start/stop methods etc that don't have locks,
> etc?

Maybe. Will keep it in mind, but most things here are subject to Major
Refactoring and being moved around for now, so "uh maybe later is easier". It
might also be straight-up doable. Not sure. :)