Gracefully handle cases when WebFrameClient::createChildFrame returns 0.

This can occur when the WebView is being closed or a navigation that
results in a cross process swap is occurring.

BUG=336014

Review URL: https://codereview.chromium.org/133233006

git-svn-id: svn://svn.chromium.org/blink/trunk@165563 bbb929c8-8fbe-4397-9dbb-9b2b20218538