[devtools] De-flake OOPIF elements panel test

Instead of waiting for multiple life-cycle 'load' events we wait for
one DOMContentLoaded event. This guarantees that the iframe is ready
with the DOM.

Expanding the iframe still happens asynchronously so the timeout is
still required. For some reason the elements panel doesn't work nicely
with `expandRecursively`, like, at all.

Note that reliability is still drastically improved when running
the test locally with multiple parallel content_shells.

R=alexrudenko@chromium.org

Fixed: 358504673
Change-Id: I601ec8629b02fb9dc75876589a2191cfd0f8dc55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6591362
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1465864}