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

This reduces the number of 'base::Bind' in //headless from 142 to 21,
and //out/Release/gen from 1011 to 466 as tracked at
http://goo.gl/LUVhDj

Bug: 714018
Change-Id: Id4ed7a54b9aab6fb649e8bf160b54595daadda88
Reviewed-on: https://chromium-review.googlesource.com/956791Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542387}