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- allow-sxg-certs-without-extension is still used by developers for
  testing.
- enable-sxg-prefetch-cache-for-navigations and
  enable-sxg-subresource-prefetching are related to SignedExchange
  Subresource prefetching, which is shipped in September. Being able to
  disable it with a flag would be useful for diagnosis, so let's keep
  them for a while.

Bug: 862003,935267,968427
Change-Id: Ice8942664b5418a1c0119f65d89eed1fa507a21f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2509153Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823075}