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This updates //crypto to use base::OnceCallback instead of legacy
base::Callback. No intended functional change is included.

This reduces the number of 'base::Bind' in //crypto from 3 to 0,
as tracked at http://goo.gl/LUVhDj

Bug: 714018
Change-Id: I020993c154a0706a262543937d9ebe506ea01c25
Reviewed-on: https://chromium-review.googlesource.com/952507Reviewed-by: Adam Langley <agl@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542030}