function charsRightIndex(string, chars) {
    var index = string.length;

    while (index-- && chars.indexOf(string.charAt(index)) > -1) {}
    return index;
  }
Used by `_.trim` and `_.trimRight` to get the index of the last character
of `string` that is not found in `chars`.

@private
@param {string} string The string to inspect.
@param {string} chars The characters to find.
@returns {number} Returns the index of the last character not found in `chars`.