VTT file with styles crashes the parser

[OrenMe]

Assigning `localName' which is read only(MDN link) causes exception:

vtt.js/lib/vtt.js

Lines 377 to 380 in c77cd38

	var element = window.document.createElement(tagName);
	element.localName = tagName;
	var name = TAG_ANNOTATION[type];
	if (name && annotation) {

I don't see any reason to keep it, just remove this.