Implement TOGGLE_EXPANSION action for the ARC notifications.

ui/arc/notification/arc_notification_item.cc

Index: ui/arc/notification/arc_notification_item.cc
diff --git a/ui/arc/notification/arc_notification_item.cc b/ui/arc/notification/arc_notification_item.cc
index 5beb310f5232186ab1ec73bb8d811844706edf14..eac63a34260cf5393ca3da2b7db109c3dfe430f6 100644
--- a/ui/arc/notification/arc_notification_item.cc
+++ b/ui/arc/notification/arc_notification_item.cc
@@ -279,6 +279,10 @@ bool ArcNotificationItem::IsOpeningSettingsSupported() const {
   return manager_->IsOpeningSettingsSupported();
 }
 
+void ArcNotificationItem::ToggleExpansion() {
+  manager_->SendNotificationToggleExpansionOnChrome(notification_key_);
+}
+
 // Converts from Android notification priority to Chrome notification priority.
 // On Android, PRIORITY_DEFAULT does not pop up, so this maps PRIORITY_DEFAULT
 // to Chrome's -1 to adapt that behavior. Also, this maps PRIORITY_LOW and _HIGH