Add ComputeSHA256DigestOfString function to chrome_cleaner/os

Bug: 830892
Change-Id: I3b900e68527f31cfd52cc27d98cfcdd3a22d001b
Reviewed-on: https://chromium-review.googlesource.com/1147497
Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579511}
5 files changed