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This cl creates the preference and layout for creating a button in
ManageSyncSettings that will be used to sign the user out using
SignOutDialogFragment

Screenshot: https://crbug.com/1092410#c1

Bug: 1092410
Change-Id: If66793a3c7f2a965c68b441a32d47718bd943c86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264338
Commit-Queue: Tanmoy Mollik <triploblastic@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790320}