Bug 1610422 part 2. Revert the bug 1609859 canges to RTCPeerConnection-createDataChannel.html. r=jgraham

This is a clean revert of the changes bug 1609859 made to
RTCPeerConnection-createDataChannel.html. It's separated out for review ease;
these changes are reinstated 2 changesets later.

Differential Revision: https://phabricator.services.mozilla.com/D60470