IDB: Extend idbindex_count WPT to cover workers in addition to window.

web_tests/external/wpt/IndexedDB/Tests currently only verify in window
environment. Extend tests to cover dedicated, shared and service
workers. Tests related to idbindex_count.htm have been extended.

Bug: 41455766

Change-Id: I392bbfd2ccb7ef6a836397a7a88624402eaffafb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5311656


Reviewed-by: Ayu Ishii <ayui@chromium.org>
Commit-Queue: Sneha Agarwal <snehagarwal@microsoft.com>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267473}