function charsStartIndex(strSymbols, chrSymbols) {
    var index = -1,
        length = strSymbols.length;

    while (++index < length && baseIndexOf(chrSymbols, strSymbols[index], 0) > -1) {}
    return index;
  }
Used by `_.trim` and `_.trimStart` to get the index of the first string symbol
that is not found in the character symbols.

@private
@param {Array} strSymbols The string symbols to inspect.
@param {Array} chrSymbols The character symbols to find.
@returns {number} Returns the index of the first unmatched string symbol.