[Feed] Add a feature flag for Feed notifications

This flag will be used to control whether to show notifications for
Feed articles.

Also, increase interest-feed-content-suggestions expiry milestone to 78.

Bug: 982483
Change-Id: I759721cd0e005612ecc40352f43e5608287591ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693816
Commit-Queue: Ian Wells <[email protected]>
Reviewed-by: Dan H <[email protected]>
Reviewed-by: Sky Malice <[email protected]>
Cr-Commit-Position: refs/heads/master@{#677057}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 5d9db58..2e12485 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1392,6 +1392,9 @@
 extern const char kExploreSitesName[];
 extern const char kExploreSitesDescription[];
 
+extern const char kInterestFeedNotificationsName[];
+extern const char kInterestFeedNotificationsDescription[];
+
 extern const char kForegroundNotificationManagerName[];
 extern const char kForegroundNotificationManagerDescription[];