Make DBusStatistics only run on the main thread

dbus/dbus_statistics.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DBUS_DBUS_STATISTICS_H_
 #define DBUS_DBUS_STATISTICS_H_
 
 #include <string>
 
 #include "dbus/dbus_export.h"
 
+// The functions defined here are used to gather DBus statistics, and
+// provide them in a format convenient for debugging. These functions are only
+// valid when called from the main thread (the thread which Initialize() was
+// called from). Calls from other threads will be ignored.
+
 namespace dbus {
 namespace statistics {
 
 // Enum to specify what level of detail to show in GetAsString
 enum ShowInString {
   SHOW_SERVICE = 0,  // Service totals only
   SHOW_INTERFACE = 1,  // Service + interface totals
   SHOW_METHOD = 2,  // Service + interface + method totals
 };
 
@@ skipping 43 matching lines @@
                                  const std::string& method,
                                  int* sent,
                                  int* received,
                                  int* blocking);
 }  // namespace testing
 
 }  // namespace statistics
 }  // namespace dbus
 
 #endif  // DBUS_DBUS_STATISTICS_H_