Add UMA histogram data representing the Synced Notifications feature.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 3d8917cca4f5ba1d5c9d36ad495aa1e42c79d375..fffcb58977233707aa58c6f71e61b9fa30c8f0f6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17866,6 +17866,14 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="SyncedNotification.Actions"
+    enum="SyncedNotificationActionType">
+  <summary>
+    The actions taken on synced notifications, recorded every time they happen.
+    This histogram will record every single event that happens separately.
+  </summary>
+</histogram>
+
 <histogram name="Tab.AgeUponRestoreFromColdStart" units="minutes">
   <summary>
     Age (time since the last display in previous sessions) of a tab being
@@ -27240,6 +27248,14 @@ other types of suffix sets.
   <int value="3" label="Attempted"/>
 </enum>
 
+<enum name="SyncedNotificationActionType" type="int">
+  <int value="0" label="Unknown"/>
+  <int value="1" label="Notification clicked"/>
+  <int value="2" label="Notification button clicked"/>
+  <int value="3" label="Notification closed by user"/>
+  <int value="4" label="Notification closed by system"/>
+</enum>
+
 <enum name="TabRestoreResult" type="int">
   <int value="0" label="Failure (other)"/>
   <int value="1" label="Success"/>