-webkit-text-size-adjust
It specifies that if you(author) want's to resize the text when the browser page is zoomed. Most of the times it is better to set it to auto or 100% as the web browser will take care of how resizing of text is to be done.
Note : Mozilla devlopers page says its a non-standard
This article will help you more.
Interesting note : Developers using Twitter Bootstrap may be interested to know that Bootstrap uses ‑webkit‑text‑size‑adjust: 100% out of the box : Source Link
Edit
Chrome on android phones uses font boosting, so -webkit-text-size-adjust is being ignored no matter what value you set. You can disable font boosting it by setting max-height to something large(100000px or so) see this bug.