LiveEdit main entry point: changes a script text to a new string.
function SetScriptSource(script, new_source, preview_only, change_log) {
    var old_source = script.source;
    var diff = CompareStrings(old_source, new_source);
    return ApplyPatchMultiChunk(script, diff, new_source, preview_only,
        change_log);
  }