Enable the cors/ wpt subset

Fixes #3781
auto merge of #3952 : andrewguertin/servo/wpt-cors, r=Manishearth

Fixes #3781 

I had to make a small fix to wptrunner's wptupdate, but otherwise the instructions at https://github.com/servo/servo/blob/master/tests/wpt/README.md#updating-test-expectations were good and this was very straightforward.