Implement URLUtils and URL interface

This does not implement the URLQuery part of the URL spec yet.

This moves the URLUtils WebIDL attributes from HTMLAnchorElement
to URLUtils and let both URL and HTMLAnchorElement implement
URLUtils.

This allows creating new URL objects using `new URL`, which can
later be mutated using the same property names as provided by
HTMLAnchorElement and Location.

BUG=303152

Review URL: https://codereview.chromium.org/24066010

git-svn-id: svn://svn.chromium.org/blink/trunk@159337 bbb929c8-8fbe-4397-9dbb-9b2b20218538