URLUtils: Use ImplementedAs for toString

This reverts the revert 589b2dfdfcf7b0a59a46398efda0693fc6e1b5d5.

BUG=None
TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/27179005

git-svn-id: svn://svn.chromium.org/blink/trunk@159597 bbb929c8-8fbe-4397-9dbb-9b2b20218538