merge_pr_60222

[focusgroup] Change focusGroup IDL from DOMString to DOMTokenList

Change the focusGroup DOM property from a reflected DOMString to a
DOMTokenList, matching the latest iteration of the HTML spec PR and OpenUI explainer.

Bug: 4021071
Change-Id: Idce8741cf8ecd0d241cf58d7419af323b173c29c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7871961
Commit-Queue: Jacques Newman janewman@microsoft.com
Reviewed-by: Mason Freed masonf@chromium.org
Cr-Commit-Position: refs/heads/main@{#1637254}