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This change:
1) Fixes two errors in flag-metadata.json, both instances of:
     "owners": [ "foo,bar" ]
   where
     "owners": [ "foo", "bar" ]
   was meant.
2) Adds a new unit test, AboutFlagsTest.OwnersLookValid, that detects
   this mistake and another couple of common syntax mistakes in owner
   metadata.

Bug: None
Change-Id: I944efd9c95fcb6a2ccdce2b174b542ddf3e451d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912803
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714977}