Migrate TaskFinishedCallback to OnceCallback.

Also updated DownloadServiceImpl's pending tasks and actions to use
OnceClosure and fixed lint errors in modified files.

Bug: 714018
Change-Id: I3efdae52913657519dfca2bd1aa7b46aa98ae45d
Reviewed-on: https://chromium-review.googlesource.com/1117311
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571573}
22 files changed