Fold IPCSupportInitializer::ForceShutdown() with ShutDown().

Fold IPCSupportInitializer::ForceShutdown() with ShutDown().

A race is possible on browser shut down where the UI thread releases
its last |ScopedIPCSupport| which then causes ShutDown() to call
ForceShutdown() since the last reference is released. However, at the
point ForceShutdown() is called, |lock_| is not held and so a utility
process being created on the IO thead can call Init() and assume it has
a reference to the IPC support. However, |shutting_down_| hasn't been
set yet, so there's no way to cancel the shutdown process. This causes
|ChannelManager| internally to post a task to the IO thread with a
reference to itself, even though it is about to be destroyed.

BUG=542069
Committed: https://crrev.com/55eeeffd666bd30f20bf596f117ea1a101d4b202
Cr-Commit-Position: refs/heads/master@{#354924}

[Anand Mistry (off Chromium), 2015-10-19 02:58:26]

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

[Ken Rockot(use gerrit already), 2015-10-19 21:54:46]

lgtm

[Anand Mistry (off Chromium), 2015-10-19 22:02:23]

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

[Anand Mistry (off Chromium), 2015-10-19 22:02:23]

jam@chromium.org: Because OWNERS.

[jam, 2015-10-19 22:54:31]

On 2015/10/19 22:02:23, Anand Mistry wrote:
> mailto:jam@chromium.org: Because OWNERS.

Anand: can you add yourself and Ken to ipc/mojo/OWNERS? then with that, lgtm

[Anand Mistry (off Chromium), 2015-10-19 23:04:33]

On 2015/10/19 22:54:31, jam wrote:
> On 2015/10/19 22:02:23, Anand Mistry wrote:
> > mailto:jam@chromium.org: Because OWNERS.
> 
> Anand: can you add yourself and Ken to ipc/mojo/OWNERS? then with that, lgtm

Done, thanks.

[Anand Mistry (off Chromium), 2015-10-19 23:12:01]

The CQ bit was checked by amistry@chromium.org

[Anand Mistry (off Chromium), 2015-10-19 23:12:02]

The patchset sent to the CQ was uploaded after l-g-t-m from rockot@chromium.org,
jam@chromium.org
Link to the patchset: https://codereview.chromium.org/1412733006/#ps20001
(title: "Add myself and Ken to owners.")

[commit-bot: I haz the power, 2015-10-19 23:13:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1412733006/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1412733006/20001

[commit-bot: I haz the power, 2015-10-20 00:13:23]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-20 00:14:38]

Patchset 2 (id:??) landed as
https://crrev.com/55eeeffd666bd30f20bf596f117ea1a101d4b202
Cr-Commit-Position: refs/heads/master@{#354924}