[iOS] Adds everyone as BUILD.gn owner for flags

about_flags.* is already owned by everyone to allow for easy addition
of flags. This CL is doing the same for its BUILD.gn file to allow for
easier addition of new flags.

Bug: None
Change-Id: I343351d361620b1977951d0769db54cde8444b21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2385301
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803424}

ios/chrome/browser/flags/OWNERS

 # Only for adding and removing flags.
+per-file BUILD.gn=*
 per-file about_flags.*=*
 per-file ios_chrome_flag_descriptions.*=*