Shutdown the It2Me host cleanly.

Shutdown the It2Me host cleanly.

Previously It2Me host was shutting down before letting all shutdown
tasks finish, which would often cause crashes.
Now NativeMessagingPipe is responsible for host and channel deletion
and the task runner for the main thread doesn't stop until the host
is destroyed.

BUG=453016
Committed: https://crrev.com/1a7589308f5cda5f58fa70a515962add2a33f7a3
Cr-Commit-Position: refs/heads/master@{#313967}

[Sergey Ulanov, 2015-01-30 01:50:50]

sergeyu@chromium.org changed reviewers:
+ kelvinp@chromium.org

[kelvinp, 2015-01-30 19:01:30]

lgtm

[Sergey Ulanov, 2015-01-30 19:25:39]

The CQ bit was checked by sergeyu@chromium.org

[commit-bot: I haz the power, 2015-01-30 19:26:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/887853002/1

[commit-bot: I haz the power, 2015-01-30 20:02:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-30 20:04:45]

Patchset 1 (id:??) landed as
https://crrev.com/1a7589308f5cda5f58fa70a515962add2a33f7a3
Cr-Commit-Position: refs/heads/master@{#313967}