chrome: put all flags into flag metadata

This change:

1) Adds every current flag to flag-metadata.json
2) Enables AboutFlagsTest.EveryFlagHasMetadata to ensure that no future flags
   without metadata are added

Note that every flag has been set to expire in M76 per
<https://sites.google.com/a/chromium.org/dev/flag-ownership>.

Bug: 897809
Change-Id: I67ab00e0462a50bb1a1484154f225c6e82d46097
Reviewed-on: https://chromium-review.googlesource.com/c/1335867
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608789}
3 files changed