Update tests to work with new behavior.

This feature is shipping in Firefox 54, so compatibility risk is low.

R=littledan@chromium.org, adamk@chromium.org, caitp@igalia.com
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Bug: v8:4958
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Ib16d19468cf935f961d7bcd856ebbeb5692d3e61
Reviewed-on: https://chromium-review.googlesource.com/546941
Commit-Queue: Josh Wolfe <jwolfe@igalia.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50178}