URLUtils: Use ImplementedAs in the idl

This is so that we do not need to have a toString in the C++ code.

BUG=None
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/26960005

git-svn-id: svn://svn.chromium.org/blink/trunk@159476 bbb929c8-8fbe-4397-9dbb-9b2b20218538