Add timeout extension to notify-event-iframe.https.html

This test appears to be timing out often on buildbots. I think
it just needs a timeout extension, given that there are multiple
test cases in one file. If there's still significant flakes after
a day or two, I'll take a closer look at the test behavior and
see if there's anything else that I can fix.

Bug: 331799346
Change-Id: Ifc90a5aec61f84b623b6a2f1edf7c881e5a56ad2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405801


Reviewed-by: Garrett Tanzer <gtanzer@chromium.org>
Commit-Queue: Andrew Verge <averge@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280168}