function charsLeftIndex(string, chars) {
    var index = -1,
        length = string.length;

    while (++index < length && chars.indexOf(string.charAt(index)) > -1) {}
    return index;
  }
Used by `_.trim` and `_.trimLeft` to get the index of the first 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 first character not found in `chars`.