function documentFragment(children, height, depth, maxFontSize) {
	    this.children = children || [];
	    this.height = height || 0;
	    this.depth = depth || 0;
	    this.maxFontSize = maxFontSize || 0;
	}
This function is a convenience function for looking up information in the
metricMap table. It takes a character as a string, and a style.

Note: the `width` property may be undefined if fontMetricsData.js wasn't
built using `Make extended_metrics`.