Mojo: Add the ability to hook up a channel to the embedder API.

Mojo: Add the ability to hook up a channel to the embedder API.

That is, starting with an OS "channel" (e.g., socket), you can tell the
system to set up to send messages over it. It'll provide an initial
"bootstrap" message pipe, from which you'll eventually be able to bring
up more message pipes multiplexed on that channel (once you can pass
handles in messages over the channel).

Also:
- Make a base class for tests that need an IO thread.
- Add a way for test embedders to shut down the global instance.

R=darin@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245251

[viettrungluu, 2014-01-16 01:17:24]

Notes:
- I definitely want to make a separate directory for embedder stuff and move
stuff there -- it's getting messy. I'll do that next.
- The new test will sometimes crash; I'm 99.9% sure that this is revealing a
pre-existing race condition. I'll also need to look at that.
- As a TODO indicates, there's a more benign (also pre-existing) race condition
(when both sides "simultaneously" close message pipe handles). I'll fix this
separately too.

[darin (slow to review), 2014-01-16 08:44:15]

LGTM

[viettrungluu, 2014-01-16 18:10:56]

Committed patchset #3 manually as r245251 (presubmit successful).