Implement new/revised popover=hint behavior

[chromium-wpt-export-bot]

See this discussion for more context:

whatwg/html#12304
whatwg/html#12345

This CL implements a simplified behavioral model for popover=hint and
popover=auto, cleanly separating the autoRootedPopoverStack from the
hintRootedPopoverStack and introducing a hintStackParent to track
where the hint stack branches off the auto stack.

The new behavior resolves the following inconsistencies (gated behind
the PopoverHintNewBehavior experimental runtime flag):

1. Opening a hint popover will not hide unrelated auto popovers.
2. Opening a hint popover closes only other non-ancestor hint
   popovers.
3. Clicking outside consistently closes both auto and hint popovers.
4. Hiding an auto popover closes only its child popovers.
5. Opening an auto popover inside a hint popover is disallowed and
   will fail.

A new WPT test (popover-hint-hierarchy.html) is added to explicitly
assert these 5 rules, and existing popover WPTs are updated to reflect
the new behavior. A virtual test suite (popover-hint-old-behavior)
is also added to continue testing the legacy behavior. I'm not sure
how to best handle the changed tests (and I guess the new test too)
in the context of a spec PR that is forthcoming. I'd like to be able
to publish (as a WPT PR at least) the changes, to help with the PR
effort. Suggestions appreciated.

A note about the diff: I made all of the feature flag checks look like
if (!feature enabled) { in the hopes that the old code would show
as unchanged in the diff, but gerrit's diff algorithm isn't great.
It's at least a little better like this than if I put the new code
first, so I left it.

Bug: 4990199
Change-Id: I41d8d96be71ba60ec5e7aa640c935258d1e33431
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7727959
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1615186}

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

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