When the user specifies serif, sans-serif or monospace as the font-family on iOS, we should obtain a system font, for example using the UIFont.systemFontOfSize(...) method.

Currently they are hard-coded as:

var DEFAULT_SERIF = "Times New Roman";
var DEFAULT_SANS_SERIF = "Helvetica";
var DEFAULT_MONOSPACE = "Courier New";

This does not allow the use of fonts such as Helvetica Neue and San Francisco.

Slightly related to #1862