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Currently, feature initialization looks like this:

1. The pref store is initialized (which contains saved flag states)
2. A FlagsState instance is initialized from that prefs store
  a. FlagsState computes which switches and features should be in what
     states based on flags
  b. FlagsState excludes switches and features from flags that are
     currently expired
3. A FeatureList is initialized from that FlagsState

Unfortunately, deciding whether a flag is expired or not requires
knowing the state of the various TemporaryUnexpireFlagsM$M features,
which aren't initialized until step 3, so step 2b doesn't actually work
properly. During step 2b, every unexpiry feature is treated as if it was
in its default state (i.e. disabled), which means that every flag whose
expiration has passed is treated as expired, regardless of the state of
the unexpiry flag. As a result of *that*, when the FeatureList is
initialized at step 3, any such flag's configured value gets ignored.

The fix for this is deeply unfortunate: at step 2b, add a special case
in checking whether a flag should be considered expired or not.
Specifically, consider not just the state of the unexpiry feature (which
may not have been initialized yet) but also the state of the backing
unexpiry *flag*. To enable this, pass a FlagsStorage into the
FlagsState::Delegate.

Change-Id: Ia2b6423357d24318baaeb3ca34208d1a2f1c36aa
Bug: 1101828
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302925Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794557}