[sync] Expose whether recoverability is degraded in profile menu

The goal of the UI is to notify the user if the recoverability of the
account that holds the sync data is not good enough, and the user may
run into data loss. To fix this case, the user can press the button
in the profile menu. The actual action corresponding to this button
is not introduced yet in this patch but instead TODOs have been added,
to be addressed in follow-up patches.

All logic is behind a feature toggle in lower layers
(SyncServiceCrypto) and disabled by default.

Change-Id: I86977e710b6109d86f2384b235dbeafaebd762fb
Bug: 1081649
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461326
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826301}