Revert node.contains to the original behavior.

The node.contains changed the behavior in
https://chromiumcodereview.appspot.com/21123005

We don't have any spec for that.
Until we have a wider consensus, it should be better to revert the change.
It is not wise to leave this change as is because it doesn't follow the semantics of node.contains defined in DOM living standard.

http://dom.spec.whatwg.org/#dom-node-contains

This is discussed on this W3 bug:
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=22141>

Review URL: https://codereview.chromium.org/197283016

git-svn-id: svn://svn.chromium.org/blink/trunk@169296 bbb929c8-8fbe-4397-9dbb-9b2b20218538