Add a browser test to verify that double-closing a notification does not crash.

Add a browser test to verify that double-closing a notification does not crash.

BUG=435201
Committed: https://crrev.com/c50e90e50d4d105f0d2260385a2090d9108c9fa3
Cr-Commit-Position: refs/heads/master@{#305495}

[Peter Beverloo, 2014-11-21 13:39:45]

peter@chromium.org changed reviewers:
+ mukai@chromium.org

[Peter Beverloo, 2014-11-21 13:39:46]

Jun: PTAL.

This depends on the following Blink patch:
  https://codereview.chromium.org/745713003/

[Mike West, 2014-11-21 13:52:10]

mkwst@chromium.org changed reviewers:
+ mkwst@chromium.org

[Mike West, 2014-11-21 13:52:11]

LGTM, thanks for uploading this followup to the Blink-side patch.

[Jun Mukai, 2014-11-21 21:14:05]

lgtm

[Peter Beverloo, 2014-11-24 12:22:04]

The CQ bit was checked by peter@chromium.org

[commit-bot: I haz the power, 2014-11-24 12:22:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/752533002/1

[commit-bot: I haz the power, 2014-11-24 13:20:30]

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

[commit-bot: I haz the power, 2014-11-24 13:20:31]

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

[Peter Beverloo, 2014-11-24 20:53:11]

The CQ bit was checked by peter@chromium.org

[commit-bot: I haz the power, 2014-11-24 20:53:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/752533002/1

[commit-bot: I haz the power, 2014-11-24 21:26:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-24 21:27:57]

Patchset 1 (id:??) landed as
https://crrev.com/c50e90e50d4d105f0d2260385a2090d9108c9fa3
Cr-Commit-Position: refs/heads/master@{#305495}