URLUtils: Use ImplementedAs for toString

This is a second try. The first time the builders had some snafu

Original: https://src.chromium.org/viewvc/blink?revision=159476&view=revision
Revert:   https://src.chromium.org/viewvc/blink?revision=159477&view=revision

BUG=None
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/27053003

git-svn-id: svn://svn.chromium.org/blink/trunk@159503 bbb929c8-8fbe-4397-9dbb-9b2b20218538