Add Web Platform Test for user-reported difference
in behavior between Safari and Chrome/Edge/Firefox.

Correct behavior confirmed in
https://github.com/w3c/csswg-drafts/issues/1933

BUG=779942

Change-Id: I808b39a7cb678a6efe09991697abdabef54384f9
Reviewed-on: https://chromium-review.googlesource.com/755015
Reviewed-by: Bugs Nash <[email protected]>
Commit-Queue: Eric Willigers <[email protected]>
Cr-Commit-Position: refs/heads/master@{#514284}