[e] (0) Rename API referrer source to just referrer source, for fetch
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=27032
Affected topics: DOM APIs, Workers

git-svn-id: https://svn.whatwg.org/webapps@8846 340c8d12-0b0e-0410-8428-c7bf67bfef74