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

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

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@168395 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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