Crash when the widget is 0 in RenderWidget::paintContents.

Hopefully this will give us more clues as to why this would ever happen.

BUG=412714

Review URL: https://codereview.chromium.org/582183002

git-svn-id: svn://svn.chromium.org/blink/trunk@182273 bbb929c8-8fbe-4397-9dbb-9b2b20218538