Ensure initial about:blank Doc's initialness is not carried over

[chromium-wpt-export-bot]

This CL adds a test to ensure that the initial about:blank Document's
initial-ness is not carried over to another about:blank Document, when
navigating from the initial about:blank Document to about:blank.

This CL also fixes or cleans up some documentation in related tests.

R=rakina

Bug: N/A
Change-Id: If5788ecb534b61b541551e57cdd2e37a8df26ea5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7101623
Reviewed-by: Rakina Zata Amni <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1538831}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.

[jonathan-j-lee]

WPT Command: python3 ./wpt run --channel=nightly --verify --verify-no-chaos-mode --verify-repeat-loop=0 --verify-repeat-restart=10 --github-checks-text-file=/home/test/artifacts/checkrun.md --affected base_head --log-mach-level=info --log-mach=- -y --no-pause --no-restart-on-unexpected --install-fonts --verify-log-full --no-headless --binary=/home/test/build/firefox/firefox firefox

Some affected tests had inconsistent (flaky) results:

Unstable results

Test	Subtest	Results	Messages
/html/browsers/browsing-the-web/navigating-across-documents/initial-empty-document/iframe-src-204-fragment.html	src	FAIL: 3/10, PASS: 7/10	assert_equals: history.length must not change after normal navigation from initial empty document expected 1 but got 2
/html/browsers/browsing-the-web/navigating-across-documents/initial-empty-document/iframe-src-204-fragment.html	location.href	FAIL: 3/10, PASS: 7/10	assert_equals: history.length must not change after fragment navigation on initial empty document expected 2 but got 1
/html/browsers/browsing-the-web/navigating-across-documents/initial-empty-document/iframe-src-204-fragment.html	location.assign	FAIL: 3/10, PASS: 7/10	assert_equals: Inserting iframe with src set to URL that doesn't load a new document must not change history.length expected 2 but got 1

These may be pre-existing or new flakes. Please try to reproduce (see the above WPT command, though some flags may not be needed when running locally) and determine if your change introduced the flake. If you are unable to reproduce the problem, please tag @web-platform-tests/wpt-core-team in a comment for help.

[jonathan-j-lee]

@jcscottiii or @DanielRyanSmith: Filed https://crbug.com/457445579. Please merge, thanks!