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/1250309Reviewed-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}

chrome/browser/ui/startup/bad_flags_prompt.cc

@@ -117,7 +117,7 @@ static const char* kBadFlags[] = {
 // Dangerous feature flags in about:flags for which to display a warning that
 // "stability and security will suffer".
 static const base::Feature* kBadFeatureFlagsInAboutFlags[] = {
-    &features::kSignedHTTPExchange,
+    &features::kAllowSignedHTTPExchangeCertsWithoutExtension,
 #if defined(OS_ANDROID)
     &chrome::android::kCommandLineOnNonRooted,
 #endif  // OS_ANDROID