Properly handle a NULL webwidget returned from WebViewClient::createPopupMenu

Properly handle a NULL webwidget returned from WebViewClient::createPopupMenu

The code path resulting in a NULL here currently segfaults, but will return
NULL after the Chromium-side patch https://codereview.chromium.org/185653006/

BUG=347901
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168395

[adamk, 2014-03-03 19:27:53]

Not sure how to write a test for this...

[jamesr, 2014-03-03 20:17:39]

lgtm

[adamk, 2014-03-03 20:29:49]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2014-03-03 20:30:26]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/adamk@chromium.org/183833014/20001

[commit-bot: I haz the power, 2014-03-03 23:20:50]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/adamk@chromium.org/183833014/20001

[commit-bot: I haz the power, 2014-03-04 14:08:41]

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

[commit-bot: I haz the power, 2014-03-04 14:08:41]

Retried try job too often on win_layout for step(s) webkit_lint
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_layout...

[adamk, 2014-03-04 17:13:54]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2014-03-04 17:14:12]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/adamk@chromium.org/183833014/20001

[commit-bot: I haz the power, 2014-03-04 17:14:30]

Change committed as 168395