Use DOMParser in ve.createDocumentFromHtml() if available

This speeds up the tests significantly, at least in User Agents
that support DOMParser:

VE core tests in Chrome 33: 12.5s -> 5.2s
VE-MW tests in Chrome 33: 32.9s -> 20.0s
VE core tests in PhantomJS: unchanged at 9.4s (no DOMParser support)

Bug: 57586
Change-Id: Ie41cd0458e1814576fc91486208d4c81eef4b514