Enable srcset support in HTMLImageElement

The `srcset` attribute enables authors to adapt image resources to the device's
display characteristics.
It is specified at http://www.whatwg.org/specs/web-apps/current-work/#attr-img-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.

BUG=284722

Review URL: https://chromiumcodereview.appspot.com/23861003

git-svn-id: svn://svn.chromium.org/blink/trunk@158158 bbb929c8-8fbe-4397-9dbb-9b2b20218538