Implement DOMPoint of geometry interfaces.

The DOMPoint can represent a 2D point or 3D point.
This will replace non-standard WebKitPoint in the future.

This CL is dependent on:
https://crrev.com/404473004/

The specification:
http://dev.w3.org/fxtf/geometry/

Intent to Implement:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/V_bJNtOg0oM

BUG=388780

Review URL: https://codereview.chromium.org/404803002

git-svn-id: svn://svn.chromium.org/blink/trunk@178747 bbb929c8-8fbe-4397-9dbb-9b2b20218538