Add method to Validate content in the downloaded file

This CL allows us to validate some content in the downloaded file.
This allows us to resume the download by validating a small portion
of downloaded data when strong validators are not present.

BUG=965215

Change-Id: I19513fcda268228a17507f8cb761c12a4f402fde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621026
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661800}
5 files changed