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This makes the close-x keyboard focusable in most cases. This serves
a11y users who can detect its presence and use it, like other buttons,
without the Esc shortcut.

Adds the Close accessibility name back under Windows where the tooltip
is removed in favor of ones generated by the platform from HTCLOSE.

Bug: chromium:1019873, chromium:1049667, chromium:1049722
Change-Id: Ib33b992c1085d9368cc7573785b21e1ff05a9a3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2047454
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740499}