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/about_flags.cc b/chrome/browser/about_flags.cc
index d49028e1..c5457c0 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -6436,6 +6436,14 @@
      FEATURE_VALUE_TYPE(features::kSeparatePointingStickSettings)},
 #endif
 
+#if !defined(OS_ANDROID)
+    {"mute-notifications-during-screen-share",
+     flag_descriptions::kMuteNotificationsDuringScreenShareName,
+     flag_descriptions::kMuteNotificationsDuringScreenShareDescription,
+     kOsDesktop,
+     FEATURE_VALUE_TYPE(features::kMuteNotificationsDuringScreenShare)},
+#endif  // !defined(OS_ANDROID)
+
     // 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