[giow] (0) Prevent microtasks from running reentrantly via the parser.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24361
Affected topics: HTML Syntax and Parsing

git-svn-id: http://svn.whatwg.org/webapps@8507 340c8d12-0b0e-0410-8428-c7bf67bfef74