Checked validity of status_widget before calling IsActive method.

Checked validity of status_widget before calling IsActive method.
Rest of the code seem to suggest that we always check validity of status_area_widget() pointer before calling method on it.
Also a specific comment in the code "status_area_widget() may be NULL during the shutdown." seems to suggest status_area_widget being NULL is expected.
shelf_ could have been a possible culprit here, but before we reach this particular statment, we have already called methods on it, so possibility of status_area_widget() being culprit is more. This bug doesn't repro on my machine so have to go with guess work.

BUG=365631
R=skuhne@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=265790

[Mr4D (OOO till 08-26), 2014-04-23 23:18:51]

lgtm

[Shrikant Kelkar, 2014-04-23 23:20:06]

The CQ bit was checked by shrikant@chromium.org

[commit-bot: I haz the power, 2014-04-23 23:21:09]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/shrikant@chromium.org/251343003/1

[commit-bot: I haz the power, 2014-04-24 00:09:24]

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

[commit-bot: I haz the power, 2014-04-24 00:09:25]

Try jobs failed on following builders:
  tryserver.chromium on win_chromium_rel

[Shrikant Kelkar, 2014-04-24 00:26:46]

Committed patchset #1 manually as r265790 (presubmit successful).