Implement URLUtilsReadOnly and let WorkerLocation use that

This splits the implementation into DOMURLUtils and DOMURLUtilsReadOnly

This way we are now sharing the implementation of the URL related
getters.

BUG=303152
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/26839003

git-svn-id: svn://svn.chromium.org/blink/trunk@159464 bbb929c8-8fbe-4397-9dbb-9b2b20218538