Make DBusStatistics only run on the main thread

dbus/dbus_statistics.h

Index: dbus/dbus_statistics.h
diff --git a/dbus/dbus_statistics.h b/dbus/dbus_statistics.h
index 2bd9fd9baf706a5de556a3543eab063852ddb063..e0355582aab1c1a865dc1e8cbadfdcba24f7be9e 100644
--- a/dbus/dbus_statistics.h
+++ b/dbus/dbus_statistics.h
@@ -9,6 +9,11 @@
 
 #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 {