Put the ChildFrameDisconnector in its own file

This doesn't need to be in ContainerNodeAlgorithms, lets put it in its
own file. I also made the methods out of line. In the common case that's
only one extra function call per removeChild which seems totally fine.

I also renamed assertConnectedSubrameCountIsConsistent to
checkConnectedSubrameCountIsConsistent which was a review comment from
ap@webkit.org when I originally added the subframeCount logic that I hadn't 
gotten around to before.

Review URL: https://codereview.chromium.org/294193009

git-svn-id: svn://svn.chromium.org/blink/trunk@174732 bbb929c8-8fbe-4397-9dbb-9b2b20218538