function trimmedEndIndex(string) {
  var index = string.length;

  while (index-- && reWhitespace.test(string.charAt(index))) {}
  return index;
}
Used by `_.trim` and `_.trimEnd` to get the index of the last non-whitespace
character of `string`.

@private
@param {string} string The string to inspect.
@returns {number} Returns the index of the last non-whitespace character.