DialogClientView: Handle nested close requests gracefully.

DialogClientView: Handle nested close requests gracefully.

This fixes a regression from r459382 where clicking a "Learn More" link
on the NTP override bubble can spew out tabs. The problem is that opening
a tab may defocus the bubble, triggering a Close(). The extension bubble
client must still call close

BUG=712545

[tapted, 2017-04-19 02:56:09]

The CQ bit was checked by tapted@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-19 02:56:35]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-19 03:29:56]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-19 03:29:57]

Dry run: Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[tapted, 2017-04-19 03:34:40]

Description was changed from

==========
DialogClientView: handle nested close requests.

BUG=712545
==========

to

==========
DialogClientView: Gracefully handle nested close requests.

BUG=712545
==========

[tapted, 2017-04-19 03:34:57]

Description was changed from

==========
DialogClientView: Gracefully handle nested close requests.

BUG=712545
==========

to

==========
DialogClientView: Handle nested close requests gracefully.

BUG=712545
==========

[tapted, 2017-04-19 03:51:26]

Description was changed from

==========
DialogClientView: Handle nested close requests gracefully.

BUG=712545
==========

to

==========
DialogClientView: Handle nested close requests gracefully.

This fixes a regression from r459382 where clicking a "Learn More" link
on the NTP override bubble can spew out tabs. The problem is that opening
a tab may defocus the bubble, triggering a Close(). The extension bubble
client must still call close

BUG=712545
==========

[tapted, 2017-04-19 03:54:14]

The CQ bit was checked by tapted@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-19 03:54:24]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-19 04:44:57]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-19 04:44:58]

Dry run: This issue passed the CQ dry run.