Mojo: Remove DCHECK disallowing passing of a remote message pipe.

Mojo: Remove DCHECK disallowing passing of a remote message pipe.

This will make it set up a proxy message pipe, and is a stopgap measure
until we have proper message pipe passing (across multiple
channels/processes).

I don't know what bugs there might be, though I added a basic test
(across a single remote message pipe and back). I should add yet more
tests.

R=davemoore@chromium.org

Committed: https://crrev.com/fe9922293ff4500b2701880877b33ee70d90b0ea
Cr-Commit-Position: refs/heads/master@{#294686}

[DaveMoore, 2014-09-12 22:01:38]

lgtm

[viettrungluu, 2014-09-12 22:06:26]

The CQ bit was checked by viettrungluu@chromium.org

[commit-bot: I haz the power, 2014-09-12 22:09:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/566783002/1

[commit-bot: I haz the power, 2014-09-12 22:53:51]

Committed patchset #1 (id:1) as 1f2246c3a5acbc7d035401f327e44d1bf8faaa95

[commit-bot: I haz the power, 2014-09-12 22:58:53]

Patchset 1 (id:??) landed as
https://crrev.com/fe9922293ff4500b2701880877b33ee70d90b0ea
Cr-Commit-Position: refs/heads/master@{#294686}