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This class has been around since the initial commit. This change makes
it not exit(1) on errors, then adds test coverage for most of it.

Bug: None
Change-Id: I5c366415c34abaa6bb90ce4657f2f94c3a45a1ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040211Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739015}