Correct 'string literal' to 'primitive strings'

Per request in:
https://codereview.chromium.org/803173003/#msg6

String literals are not the only strings with
typeof(s) == 'string'.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String

R=hansmuller@chromium.org
BUG=

Review URL: https://codereview.chromium.org/807873002