Enable srcset support in HTMLImageElement

Enable srcset support in HTMLImageElement

BUG=

The `srcset` attribute enables authors to adapt image resources to the device's
display characteristics.
It is specified at http://www.w3.org/TR/html-srcset/

This change is a port of WebKit's srcset implementation, comprised from patches
by Romain Perier <romain.perier@gmail.com>;, Dean Jackson <dino@apple.com>; and myself.

The implementation adds the srcset parsing algorithm to HTMLParserIdioms,
adds preloading support to HTMLPreloadScanner and adds srcset support to HTMLImageElement.

It implements only the 'x' qualifier of srcset, which handles DPR switching,
and has rough consensus around it.