Fixes crash for operations on non-existing notifications.

Fixes crash for operations on non-existing notifications.

The widget's Close() will happen asynchronously and sometimes those
operation can be called after the notification is removed.

BUG=232895
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=195299

[Jun Mukai, 2013-04-19 18:25:24]

ping?

[dewittj, 2013-04-19 18:48:25]

lgtm

[commit-bot: I haz the power, 2013-04-19 18:58:04]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mukai@chromium.org/14320008/1

[commit-bot: I haz the power, 2013-04-19 19:32:09]

Sorry for I got bad news for ya.
Compile failed with a clobber build on linux_chromeos_clang.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_chro...
Your code is likely broken or HEAD is junk. Please ensure your
code is not broken then alert the build sheriffs.
Look at the try server FAQ for more details.

[commit-bot: I haz the power, 2013-04-19 20:43:10]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mukai@chromium.org/14320008/14001

[commit-bot: I haz the power, 2013-04-19 21:33:34]

Retried try job too often on mac_rel for step(s) base_unittests, net_unittests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=mac_rel&nu...

[Jun Mukai, 2013-04-19 21:56:50]

Committed patchset #2 manually as r195299 (presubmit successful).