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The only use was inside a single DCHECK in DiceHeaderHelper. That's not
worth keeping the pref and all the plumbing around.

Bug: 935956
Change-Id: Ic7d0f11751cb8ce6d09472296a87493b1848e57a
Reviewed-on: https://chromium-review.googlesource.com/c/1489191Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635532}