[LegacyTLS] Add show-legacy-tls-warnings flag

This will be used to control our UI treatments for sites using legacy
TLS versions (TLS 1.0/1.1).

Bug: 989607
Change-Id: I4bc9e21248a83bdd566bdaec31e6fa8a11b9bda1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1827775
Reviewed-by: Joe DeBlasio <[email protected]>
Commit-Queue: Christopher Thompson <[email protected]>
Cr-Commit-Position: refs/heads/master@{#700461}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 2c54d4b..ad400a16 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -4619,6 +4619,12 @@
      flag_descriptions::kPercentBasedScrollingDescription, kOsAll,
      FEATURE_VALUE_TYPE(features::kPercentBasedScrolling)},
 
+#if !defined(OS_ANDROID)
+    {"show-legacy-tls-warnings", flag_descriptions::kLegacyTLSWarningsName,
+     flag_descriptions::kLegacyTLSWarningsDescription, kOsDesktop,
+     FEATURE_VALUE_TYPE(features::kLegacyTLSWarnings)},
+#endif
+
     // NOTE: Adding a new flag requires adding a corresponding entry to enum
     // "LoginCustomFlags" in tools/metrics/histograms/enums.xml. See "Flag
     // Histograms" in tools/metrics/histograms/README.md (run the