Pause notifications while presenting full screen

This adds a new NotificationBlocker that listens for screen share
events and stops notifications when a full screen is currently being
recorded.

Implemented behind a mute-notifications-during-screen-share flag.

Bug: 1131375
Change-Id: Icebcb9223614e07134ce038aef2ec2e45190c893
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421665
Commit-Queue: Richard Knoll <[email protected]>
Reviewed-by: Peter Beverloo <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#810238}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 8855c1c..cc71de0 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1848,6 +1848,9 @@
 extern const char kGlobalMediaControlsCastStartStopName[];
 extern const char kGlobalMediaControlsCastStartStopDescription[];
 
+extern const char kMuteNotificationsDuringScreenShareName[];
+extern const char kMuteNotificationsDuringScreenShareDescription[];
+
 extern const char kNtpDismissPromosName[];
 extern const char kNtpDismissPromosDescription[];