Fix use-after-free when creating and detaching iframe during load.

WebLocalFrameImpl::createChildFrame needs to keep a reference to the
WebCore::LocalFrame in case LocalFrame::init() detaches the frame.

BUG=384890
TEST=fast/loader/create-frame-in-DOMContentLoaded.html
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/339513002

git-svn-id: svn://svn.chromium.org/blink/trunk@176232 bbb929c8-8fbe-4397-9dbb-9b2b20218538