Extend idbindex_get Web platform test coverage to workers.

[chromium-wpt-export-bot]

web_tests/external/wpt/IndexedDB/ tests are only testing behavior in window environment.
Extend the tests to cover different workers (dedicated, serviceworker, sharedworker).
Tests related to idbindex_get.htm have been extended.

Bug: 41455766

Change-Id: Ib2ffb0715a5aa994a92cd22dfa36932cddb4e637
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5311293
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Commit-Queue: Sneha Agarwal <snehagarwal@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1264153}

[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 --no-headless --verify-log-full --binary=/home/test/build/firefox/firefox firefox

Some affected tests had inconsistent (flaky) results:

Unstable results

Test	Subtest	Results	Messages
/IndexedDB/idbindex_get.any.serviceworker.html		OK: 9/10, TIMEOUT: 1/10
/IndexedDB/idbindex_get.any.serviceworker.html	IDBIndex.get() - returns the record	PASS: 9/10, MISSING: 1/10
/IndexedDB/idbindex_get.any.html		OK: 9/10, TIMEOUT: 1/10
/IndexedDB/idbindex_get.any.html	IDBIndex.get() - returns the record	NOTRUN: 1/10, PASS: 9/10
/IndexedDB/idbindex_get.any.worker.html		OK: 9/10, TIMEOUT: 1/10
/IndexedDB/idbindex_get.any.worker.html	IDBIndex.get() - returns the record	NOTRUN: 1/10, PASS: 9/10

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]

@KyleJu @DanielRyanSmith Pinged bug about Firefox stability timeout. Please force merge in the meantime, thanks!