I take this.

I think we should show that warning for chrome://flags/#allow-sxg-certs-without-extension instead; we haven't done that because #allow-sxg-certs-without-extension has no effect unless SXG is enabled.

Thanks for taking
sgtm >w/o ext

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c4be2acdf728ec22df14e07a39079dcf1b673355

commit c4be2acdf728ec22df14e07a39079dcf1b673355
Author: Kunihiko Sakamoto <ksakamoto@chromium.org>
Date: Mon Oct 01 01:43:13 2018

Signed Exchange: Update the condition to show "Bad feature flags" warning

Before this patch, "You are using an unsupported feature flag..."
warning was shown when kSignedHTTPExchange feature flag is enabled.

Now that we have fully implemented the Signed Exchange verification
feature and passed security review, this patch removes that warning
for kSignedHTTPExchange.

Instead, we show that warning if
kAllowSignedHTTPExchangeCertsWithoutExtension is enabled, because it
bypasses a part of certificate check of Signed Exchange.

Bug:  890137 
Change-Id: I285a1e236e9a18d581468f7b1af561629ee222bd
Reviewed-on: https://chromium-review.googlesource.com/1250309
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595368}
[modify] https://crrev.com/c4be2acdf728ec22df14e07a39079dcf1b673355/chrome/browser/ui/startup/bad_flags_prompt.cc