Don't require root pipeline to match new pipeline

This reverts 2b0134be7bb8a4372f1a8703b5a1b114e65351f6 because it was
not correct for iframes.
auto merge of #2602 : mbrubeck/servo/revert-root-pipeline-check, r=pcwalton

This reverts 2b0134be7bb8a4372f1a8703b5a1b114e65351f6 because it was not correct for iframes.  Oops.  (Wasn't caught by tests because we still have other crashes preventing iframe testing.)  r? @pcwalton.
Allow union types to be used as return values