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Currently, when a flag expires, it is hidden from chrome://flags but still
stored inside the backend flags store, so that if the user un-expires flags
their settings are not lost. However, the code for applying flag values by
turning them into features/switches was not aware of flag expiration, so flags
that were persisted this way would continue to apply, despite the user having no
way to undo them.

This change:
1) Adds an exclusion predicate to FlagsState, to allow client classes of
   FlagsState to prevent flags from having their values applied as features or
   switches;
2) Adds an implementation of that predicate in the //chrome FlagsStateSingleton
   that checks the flag expiration state;
3) Adds unit tests to cover the new behavior added in (1);
4) Adds a new AboutFlagsBrowserTest test to validate the end-to-end behavior
   described in this commit message by:
    a) Setting an expired flag to a non-default value
    b) Restarting (simulated via separate PRE_ vs regular tests here)
    c) Checking that the flag's switch isn't in the browser command line

TBR=rohitrao@chromium.org

Bug: 1009729
Change-Id: I7d0ee2e7660fa378b9ecec3b663cc794f7e776fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879431Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709169}