Migrate FileDownloader::DownloadFinishedCallback to OnceCallback

TBR=sky@chromium.org

Bug: 714018
Change-Id: Iae2eaaf9c313d25f0f434265148126ba5098a623
Reviewed-on: https://chromium-review.googlesource.com/1104390
Commit-Queue: Mark Pilgrim <pilgrim@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568582}
4 files changed