Migrate LaunchUnitTests callers to OnceCallback

This updates callers of base::LaunchUnitTests in several directory
to use base::OnceCallback instead of legacy base::Callback.
No intended functional change is included.

This reduces the number of 'base::Bind' in //gin, //notification_helper,
//pdf, //sql, //url, and //chrome_elf from 7 to 0 as tracked at
http://goo.gl/LUVhDj

Bug: 714018
Change-Id: Ie1bf3ad1ca90f732356d4612cfd8e3e8eca8afda
Reviewed-on: https://chromium-review.googlesource.com/954882
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542743}
4 files changed