Fix posix IPC channel hanging problem, take 3

Fix posix IPC channel hanging problem.

If a channel closes right before a send call, listeners might not be notified of
the problem, which can cause hangs. This CL fixes that and adds a test that makes
sure that this does not happen in the future.

This is similar to cl/150893002, but takes a slightly different approach to how to
make sure everything happens in the right order. In particular, it avoids closing
the socket (and calling OnChannelError()) from Send().

BUG=338709
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=252430

[cpu_(ooo_6.6-7.5), 2014-02-19 20:17:51]

nice. lgtm

[Scott Hess - ex-Googler, 2014-02-19 20:51:31]

LGTM.

My understanding is that in the Send() failure case, the caller gets an error,
and probably does something useful like ignore it or short-circuit something
(and let the further caller ignore it), eventually things will unwind to the
message processor, which may continue pumping messages from the current buffer
(which is sad, but it's an inherent race condition, so probably safe enough),
and eventually runs out of buffer, and when thing select on the channel for more
data, error and done.

Close enough?

[hubbe, 2014-02-20 20:16:19]

The CQ bit was checked by hubbe@chromium.org

[commit-bot: I haz the power, 2014-02-20 20:19:54]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/hubbe@chromium.org/172773002/160001

[commit-bot: I haz the power, 2014-02-21 00:18:31]

Change committed as 252430