Mojo: Add Channel::AttachEndpoint(), which takes a ChannelEndpoint.

Mojo: Add Channel::AttachEndpoint(), which takes a ChannelEndpoint.

This will replace AttachMessagePipeEndpoint(), once I get rid of all the
calls to it. (This change is required to move the "paused message queue"
from the ProxyMessagePipeEndpoint to the ChannelEndpoint.)

Also factor out the message writing (WriteMessageNoLock()) from the
message enqueueing (EnqueueMessage()) in ChannelEndpoint. After I move
the paused message queue to ChannelEndpoint, the latter will put stuff
into said queue if it can't write.

R=darin@chromium.org

Committed: https://crrev.com/1a1d35bb2c9ec230d6a81999dbf555df290b2466
Cr-Commit-Position: refs/heads/master@{#295729}

[darin (slow to review), 2014-09-19 05:31:01]

LGTM

[viettrungluu, 2014-09-19 17:20:32]

The CQ bit was checked by viettrungluu@chromium.org

[commit-bot: I haz the power, 2014-09-19 17:21:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/584723002/1

[commit-bot: I haz the power, 2014-09-19 17:45:06]

Committed patchset #1 (id:1) as 8db05d44c2d0ece2ebfcb93c92966a7140b49aa3

[commit-bot: I haz the power, 2014-09-19 17:45:42]

Patchset 1 (id:??) landed as
https://crrev.com/1a1d35bb2c9ec230d6a81999dbf555df290b2466
Cr-Commit-Position: refs/heads/master@{#295729}