User MessageLoopForIO::WatchFileDescriptor in proces_output_watcher

User MessageLoopForIO::WatchFileDescriptor in proces_output_watcher

The main point is to avoid usage of select for observing process'
pseudo-terminal fd. Main problems with select is that it doesn't work
for fd > 1024, which started to be passed to ProcessOutputWatcher.
Added benefit of MessageLoopForIO::WatchFileDescriptor is that it's async,
so it enables us to do some clean up for process proxy thread management.

BUG=495165
TEST=Open a lot of crosh tabs (e.g. ~30). Crosh is started in all of them.

Committed: https://crrev.com/6ca50550a8136fdde5fe87d9fe483826f2f0c74e
Cr-Commit-Position: refs/heads/master@{#342716}

[tbarzic, 2015-08-04 00:54:28]

tbarzic@chromium.org changed reviewers:
+ rkc@chromium.org

[tbarzic, 2015-08-04 00:54:29]

I think you're the only person that recently looked at this code at all :)

Let me know if you don't have cycles to go through cl.

[tbarzic, 2015-08-10 17:23:33]

On 2015/08/04 00:54:29, tbarzic wrote:
> I think you're the only person that recently looked at this code at all :)
> 
> Let me know if you don't have cycles to go through cl.

ping :)

[rkc, 2015-08-10 18:11:45]

lgtm

[tbarzic, 2015-08-10 22:02:06]

The CQ bit was checked by tbarzic@chromium.org

[commit-bot: I haz the power, 2015-08-10 22:03:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1258193002/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1258193002/80001

[commit-bot: I haz the power, 2015-08-10 22:32:53]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2015-08-10 22:33:41]

Patchset 5 (id:??) landed as
https://crrev.com/6ca50550a8136fdde5fe87d9fe483826f2f0c74e
Cr-Commit-Position: refs/heads/master@{#342716}