Update web-platform-tests expected results.
Remove needless '&mut self' from WindowMethods.
Remove needless '&mut self' from DocumentMethods.
auto merge of #2556 : Ms2ger/servo/update-wpt, r=metajack
Move WebIDL unions into their own module.

This will allow multiple unions to contain the same type.
auto merge of #2558 : Ms2ger/servo/mod-unions, r=jdm

This will allow multiple unions to contain the same type.
auto merge of #2557 : saneyuki/servo/mut, r=jdm

related #2514
Partial implementation of FormData and File
fixup! Partial implementation of FormData and File