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

This reduces the number of 'base::Bind' in //gin from 8 to 1
as tracked at http://goo.gl/LUVhDj

Bug: 714018
Change-Id: I5c4bb758181c09e2d7b0e2fec5c51a4e1eec24bb
Reviewed-on: https://chromium-review.googlesource.com/952646Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541664}