page visibility idlharness test is broken

[bzbarsky]

If you implement onvisibilitychange, it will happily tell you "Unrecognized type EventHandler" and fail the test.

Looks like @plehegar forgot to add a definition of EventHandler in 47ebaea

[plehegar]

@bzbarsky I wasn't able to confirm that the fix removes the failure. If you can check and confirm the fix, that would be nice.

[bzbarsky]

Once the obvisibilitychange fix in https://bugzilla.mozilla.org/show_bug.cgi?id=1333912 lands I should be able to do that.