I thought it was because of the early return added in MediaDocument::defaultEventHandler(), so I disabled modern media controls runtime feature in GTK+ port. But the test is still failing, so I guess it's the test that is enabling that feature
For some reason I can't reproduce it locally, but it's always failing in the bots.
Created attachment 306679 [details] Patch
Committed r215178: <http://trac.webkit.org/changeset/215178>