Set |Channel::is_shutting_down_| during channel shutdown.

Set |Channel::is_shutting_down_| during channel shutdown.

This flag is set by calling |Channel::WillShutdownSoon()|, but that may
not be called in all cases (i.e. on broken connection due to renderer
crash). crrev.com/95049185be27f0da5e9d2921a1be2b91f72b7956 relies on
this flag being set on shutdown to prevent a race with
|Channel::AttachAndRunEdpoint()|.

Committed: https://crrev.com/3dd07db10b2c6c08bed268cb6447be34d6045a6e
Cr-Commit-Position: refs/heads/master@{#351013}

[Anand Mistry (off Chromium), 2015-09-28 00:25:58]

amistry@chromium.org changed reviewers:
+ rockot@chromium.org

[Ken Rockot(use gerrit already), 2015-09-28 01:06:14]

lgtm

[Anand Mistry (off Chromium), 2015-09-28 01:12:13]

The CQ bit was checked by amistry@chromium.org

[commit-bot: I haz the power, 2015-09-28 01:12:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1367383002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1367383002/1

[commit-bot: I haz the power, 2015-09-28 01:50:51]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 01:51:39]

Patchset 1 (id:??) landed as
https://crrev.com/3dd07db10b2c6c08bed268cb6447be34d6045a6e
Cr-Commit-Position: refs/heads/master@{#351013}