Migrate startup_complete_callback to OnceCallback

Bug: 714018
Change-Id: I9ecc53ebb303b1c08ca4f21f9b948fdbfa02598c
Reviewed-on: https://chromium-review.googlesource.com/1104387
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Mark Pilgrim <pilgrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568198}
4 files changed