Added native-callable JavaScript methods for getting MPU positions

These come in two flavours: window.getMpuPosCommaSeparated() return a "x,y" string (as used in the iOS app) and window.getMpuPosJson() returns a { left:n, top:n, width:n, height:n } dictionary in JSON form, for Android. (The two functions share all essential code but the formatting. )

GIA-3023
I think this part has been merged already. It's also the only div under .article__body. It could be a div.from-content-api, though.