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 f367d518d85709a37dd94801338ba851d0b47c11..bebaca5eeb2875e1b6206cd84c498f7f284cf2e3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -17739,6 +17739,24 @@ 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="SyncedNotification.PerNotificationActions"
+    enum="SyncedNotificationActionType">
+  <summary>
+    The actions taken on synced notifications, recorded once per notification,
+    when it is closed.  This differs from the Notifications.Actions histogram in
+    that multiple events of the same type on a single notification will only
+    record a single UMA event.
+  </summary>
+</histogram>
+
 <histogram name="Tab.AgeUponRestoreFromColdStart" units="minutes">
   <summary>
     Age (time since the last display in previous sessions) of a tab being
@@ -27048,6 +27066,14 @@ 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"/>