[giow] (3) Drop the 'responsible document' concept. This also changes the referrer to use when launching a nested worker to be the URL of the outer worker instead of the URL of the outer Document, which made no sense really.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=27145
Affected topics: DOM APIs, Workers

git-svn-id: https://svn.whatwg.org/webapps@8853 340c8d12-0b0e-0410-8428-c7bf67bfef74