Add flag for dark theme security interstitials

Allows the ability to control the dark theming on these interstitials.

Bug: 947840
Change-Id: I1bc6578f328f6429641ad43241aa5be46b0846e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643871
Reviewed-by: Emily Stark <[email protected]>
Commit-Queue: Edward Jung (EMEA) <[email protected]>
Cr-Commit-Position: refs/heads/master@{#669265}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index c9bd734..5c160602 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -946,6 +946,9 @@
 extern const char kScrollableTabStripName[];
 extern const char kScrollableTabStripDescription[];
 
+extern const char kSecurityInterstitialsDarkModeName[];
+extern const char kSecurityInterstitialsDarkModeDescription[];
+
 extern const char kSendTabToSelfName[];
 extern const char kSendTabToSelfDescription[];