Add string utilities for chrome_cleaner

Also adds chrome_cleaner_unittests to run the added unit tests (only
enabled on the FYI bot for now). This is a separate test suite because
it will have Chrome Cleaner specific setup added to it as more tests
are added.

Bug: 830892
Change-Id: I33181ac5ad6bba893594838bf1cb77c83cfc69c5
Reviewed-on: https://chromium-review.googlesource.com/1024972
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555207}
16 files changed