Switch URL and URLUtils interfaces over to use ScalarValueString.

R=arv@chromium.org,jsbell@chromium.org
BUG=303152

Review URL: https://codereview.chromium.org/331313011

git-svn-id: svn://svn.chromium.org/blink/trunk@176782 bbb929c8-8fbe-4397-9dbb-9b2b20218538