fork additional raw chromium a11y code

[chunhtai]

Description

These additional files are needed to implement the the accessibility library since we can't use any internal library in engine repository.
They are raw copy of the chromium code and cannot be compiled as-is. This PR means to fork those files into engine repo to serve as the base point to make the future PR easier to review.

Related Issues

Tests

I added the following tests:

files are not compiled

Checklist

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Reviewer Checklist

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Breaking Change

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[dnfield]

LGTM

[gw280]

Make sure this is merged using rebase and merge, not squash and merge