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Currently, some flags tests that need to simulate flag expiration work
by installing a custom expiration predicate that replaces the entire
body of flags::IsFlagExpired(). This is convenient but makes it
difficult to test the expiration logic itself.

This change replaces that test hook with one that allows injecting a
custom set of flag expirations, by analogy with SetFeatureEntries(). The
expiration logic then happens as normal. Tests that used to rely on a
custom expiration predicate are rewritten to control expiration in the
same way as production code would (i.e. via features).

This change is preparatory to fixing the linked bug.

Bug: 1101828
Change-Id: Ie6336c1fc1522edf2a62cd4044c121cf86ac4e67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309352
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791267}