Add multi_profile_user_count field to system_profile proto.

chrome/browser/metrics/metrics_log.h

Index: chrome/browser/metrics/metrics_log.h
diff --git a/chrome/browser/metrics/metrics_log.h b/chrome/browser/metrics/metrics_log.h
index fe06b61f03caf343887f796cef4c3929256e6ef5..d9285ffdacac977287abf7e05b7eb5d0002e8e6c 100644
--- a/chrome/browser/metrics/metrics_log.h
+++ b/chrome/browser/metrics/metrics_log.h
@@ -184,6 +184,13 @@ class MetricsLog : public MetricsLogBase {
   // This is a no-op if called on a non-Chrome OS platform.
   void WriteBluetoothProto(metrics::SystemProfileProto::Hardware* hardware);
 
+#if defined(OS_CHROMEOS)
+  // Update the number of users logged into a multi-profile session.
+  // If the number of users change while the log is open, the call invalidates
+  // the user count value.
+  void UpdateMultiProfileUserCount();
+#endif
+
   // Observes network state to provide values for SystemProfile::Network.
   MetricsNetworkObserver network_observer_;