Add DOMPointInit

To remove generic Dictionary from a DOMPoint constructor.
No changes in behavior; fast/dom/geometry-interfaces-dom-point.html
should cover this change.

Spec: http://dev.w3.org/fxtf/geometry/#DOMPoint

BUG=403150

Review URL: https://codereview.chromium.org/558163002

git-svn-id: svn://svn.chromium.org/blink/trunk@181877 bbb929c8-8fbe-4397-9dbb-9b2b20218538