Now that we have UTF8String in the WebIDL, we can remove quite a few of the
conversions. Do that, and lift the remaining string conversions up as needed.
Also deindent Servo_ComputeColor while touching it.
Most of the remaining copies are because either bug 1606994, or because they're
WebIDL attributes that we still need to serialize back as UTF-16 (bug 1606995).
Differential Revision: https://phabricator.services.mozilla.com/D58687
--HG--
extra : moz-landing-system : lando
