Migrate UnitTestLauncher to base::OnceCallback

This migrates base::LaunchUnitTests and its relatives to OnceCallback.
UnitTestLauncher is a blocker for a small number of components to
migrate to OnceCallback, but it's the last blocker for some components.

Bug: 714018
Change-Id: I0158f64201c7b4382726331df59fa4101fa1042b
Reviewed-on: https://chromium-review.googlesource.com/951023
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541421}
8 files changed