Take 2 at browser_sync::ExtensionsActivityMonitor....

chrome/browser/sync/engine/syncer_session.h

Index: chrome/browser/sync/engine/syncer_session.h
===================================================================
--- chrome/browser/sync/engine/syncer_session.h	(revision 30303)
+++ chrome/browser/sync/engine/syncer_session.h	(working copy)
@@ -22,6 +22,7 @@
 #include "chrome/browser/sync/engine/syncer_types.h"
 #include "chrome/browser/sync/engine/syncproto.h"
 #include "chrome/browser/sync/util/event_sys.h"
+#include "chrome/browser/sync/util/extensions_activity_monitor.h"
 #include "chrome/browser/sync/util/sync_types.h"
 #include "testing/gtest/include/gtest/gtest_prod.h"  // For FRIEND_TEST
 
@@ -307,6 +308,14 @@
     source_ = source;
   }
 
+  const ExtensionsActivityMonitor::Records& extensions_activity() const {
+    return extensions_activity_;
+  }
+
+  ExtensionsActivityMonitor::Records* mutable_extensions_activity() {
+    return &extensions_activity_;
+  }
+
   bool notifications_enabled() const {
     return notifications_enabled_;
   }
@@ -355,6 +364,9 @@
   // The source for initiating this syncer session.
   sync_pb::GetUpdatesCallerInfo::GET_UPDATES_SOURCE source_;
 
+  // Information about extensions activity since the last successful commit.
+  ExtensionsActivityMonitor::Records extensions_activity_;
+
   // True if notifications are enabled when this session was created.
   bool notifications_enabled_;