Update FrameLoaderClientImpl::setOpener() to handle remote frames

Originally, there wasn't enough plumbing to actually pass a remote frame
as an opener, but now core and web both use the base frame classes,
there's no need for this hack any longer.

BUG=346764

Review URL: https://codereview.chromium.org/605673002

git-svn-id: svn://svn.chromium.org/blink/trunk@182694 bbb929c8-8fbe-4397-9dbb-9b2b20218538