[giow] (2) Revamp how focus is defined, to actually handle things we've ignored before, like viewports and scrollable regions
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23475
Affected topics: CSS, Canvas, DOM APIs, HTML, Rendering

git-svn-id: http://svn.whatwg.org/webapps@8478 340c8d12-0b0e-0410-8428-c7bf67bfef74