Reland "[A11yPerformance] Use AXBitset for bool attributes in AXNodeData"

This is a reland of commit f697c0257aed084a7a892c37690e5ab85f673b87.

The original change caused a 9% regression in blink_perf.accessibility
[1]. The use of std::variant and the subsequent conditional checks make
up the majority of the regression.

Instead of std::variant, this CL introduces an abstract interface,
AXBoolStore, and two implementations (AXVectorBoolStore and
AXBitsetBoolStore) to resolve the performance issues.

The new regression is now 2% (see first Pinpoint job on Patchset 13).

The conversion (from map to bitset) during serialization and
deserialization still contributes some overhead and will be addressed in
a future CL.

[1] https://buganizer.corp.google.com/issues/426648855

Original change's description:
> Save boolean attributes in AXNodeDate with an AXBitset instead of a
> vector. This is more performant and has a smaller footprint.
>
> For serialization, the bool_attributes getter in StructTraits iterates
> through the set attributes. If an attribute is set, it's added to the
> std::map that is returned for Mojo serialization.
>
> For deserialization, the Read method in StructTraits iterates over the
> BoolAttribute map, populating entries in the AXBitset in the C++
> AXNodeData object.
>
> The choice between AXBitset and the existing std::vector implementation
> is controlled with the feature flag kAccessibilityUseAXBitset and
> std::variant.
>
> Change-Id: I82774f642c5ceeda82ac0c252842361581336f24
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6526506
> Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
> Reviewed-by: Kevin Ellis <kevers@chromium.org>
> Reviewed-by: Joe Mason <joenotcharles@google.com>
> Commit-Queue: Jocelyn Tran <jocelyntran@google.com>
> Cr-Commit-Position: refs/heads/main@{#1475843}

Bug: 426648855
Change-Id: I4c849e70675ed421127e60cab59d23930082eea6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6673956
Commit-Queue: Jocelyn Tran <jocelyntran@google.com>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1486668}