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

Review URL: https://codereview.chromium.org/183833014

git-svn-id: svn://svn.chromium.org/blink/trunk@168395 bbb929c8-8fbe-4397-9dbb-9b2b20218538