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This change adds an accessibility page that can be shown by clicking a
link on the marketing opt in screen. The link to show the accessibility
screen is only shown if the user was shown the gesture navigation screen
before getting to the marketing opt in screen.

The accessibility page contains a toggle for changing the
kAccessibilityTabletModeShelfNavigationButtonsEnabled pref.

Bug: 976949
Change-Id: Ibbb1568389b86cd336d8c4b65fe275ca2b256ce7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078740
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746692}