flags: allow anyone to edit flag-metadata.json

To achieve this, while preserving flags-team approvals for never-expiring flags,
this change:

1) Adds //chrome/browser/flag-never-expire-list.json, which contains a JSON list
   of names of flags that are permitted to never expire;
2) Adds AboutFlagsTest.OnlyPermittedFlagsNeverExpire, which asserts that the
   flags listed in flag-metadata.json with expiry_milestone -1 are a subset of
   the flags named in flag-never-expire-list.json;
3) Updates the //chrome/browser OWNERS file to make * the owner of
   flag-metadata.json and to make flags-team the owners of
   flag-never-expire-list.json.
4) Updates the flag ownership documentation to mention
   flag-never-expire-list.json.

Bug: 897809
Change-Id: I691b85d73274b6798924b2069ef9e77e29b4a991
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512613
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639504}