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This change:
1) Adds c/b/flag-metadata.json, to store metadata about flags that is needed for
   other tooling but not for the browser;
2) Adds a disabled unit test AboutFlagsTest.EveryFlagHasMetadata, which asserts
   that every flag has an entry in this file.
3) Adds a unit test AboutFlagsTest.OnlyAllowedFlagsNeverExpire, which asserts
   that only a specified set of flags have no expiry milestone.

Next steps are to make AboutFlagsTest.EveryFlagHasMetadata pass by adding
entries for all current flags, then enable the test.

Bug: 897809
Change-Id: Icbf0055cb88106e7212bd6a1f90fcf9350fc7bf4
Reviewed-on: https://chromium-review.googlesource.com/c/1309106
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607955}