Always match :focus-visible on focused element according to user preference.

(Very belated reland of https://crrev.com/c/1886075)

The :focus-visible pseudo-selector emulates the existing behaviour of
native elements like buttons by only showing a focus ring when focus is
moved using the keyboard.

This change modifies that behaviour to always match :focus-visible on
the focused element, regardless of how focus was moved (effectively
making :focus-visible equivalent to :focus) if the user has enabled the
"Highlight the object with keyboard focus when it changes" setting on
Chrome OS.

This plumbs through the ash::prefs::kAccessibilityFocusHighlightEnabled
preference on Chrome OS to a new web setting on Blink, and checks that
setting when computing whether :focus-visible matches on the focused
element.

Fixed a bug in the test which was causing leak detector to fail.

TBR=dominickng@chromium.org,ellyjones@chromium.org,falken@chromium.org,tkent@chromium.org,dmazzoni@chromium.org

Bug: 817199
Change-Id: If86d4f1c757cba0b5fd3b19ce55c656054fae671
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2198842
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Chris Hall <chrishall@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Chris Hall <chrishall@chromium.org>
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768659}