function normalizeFunctionCall(tokens, start) {
    var parenDepth = 1, end = start + 1, n = tokens.length;
    while (end < n && parenDepth) {
      // TODO: Can URLs appear in functions?
      var token = tokens[end++];
      parenDepth += (token === '(' ? 1 : token === ')' ? -1 : 0);
    }
    return end;
  }
Normalize tokens within a function call they can match against
cssSchema[propName].cssExtra.
@return the exclusive end in tokens of the function call.