[Browser Layout] Extend tests to immersive mode.

These tests only apply to ChromeOS and Mac. They verify that in
immersive mode when the toolbar/top container are revealed (when the
user mouses to the top of the screen) the visual elements render
correctly. These use a slightly different set of logic that should be
tested as a separate case.

For some reason this is being flagged as changing all
webdriver_wpt_tests, causing it to appear to flake, despite not making
any changes to these tests, hence the "validate test flakiness" tag
below.

Validate-Test-Flakiness: skip
Change-Id: I463933cd39e9ac954a60742eb5c52041b1ab8717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6974726
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519542}