GitLab

Reduces flakiness; improves test readability. Also introduces a
callback on SimpleIndexFile to signal completion of writing the index,
useful for deflaking cache serialization.

R=jkarlin@chromium.org
BUG=255775,429672

Review URL: https://codereview.chromium.org/699743002

Cr-Commit-Position: refs/heads/master@{#302653}