[giow] (0) Lay the groundwork for interruptible microtasks, mutation-observer style.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=22296
Affected topics: Canvas, DOM APIs, HTML, Video Text Tracks, Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8510 340c8d12-0b0e-0410-8428-c7bf67bfef74