Fix a potential multi-thread issue when manipulating removable storage map

base/system_monitor/system_monitor.h

Index: base/system_monitor/system_monitor.h
diff --git a/base/system_monitor/system_monitor.h b/base/system_monitor/system_monitor.h
index 48d512b708c11aa44368c05300dd9a38f29e00ed..f5224a460e2488e3cf723902f88c91c7abb3ca51 100644
--- a/base/system_monitor/system_monitor.h
+++ b/base/system_monitor/system_monitor.h
@@ -13,6 +13,7 @@
 #include "base/basictypes.h"
 #include "base/file_path.h"
 #include "base/string16.h"
+#include "base/synchronization/lock.h"
 #include "build/build_config.h"
 
 // Windows HiRes timers drain the battery faster so we need to know the battery
@@ -219,6 +220,8 @@ class BASE_EXPORT SystemMonitor {
   std::vector<id> notification_observers_;
 #endif
 
+  // For manipulating removable_storage_map_ structure.
+  mutable base::Lock removable_storage_lock_;
   // Map of all the attached removable storage devices.
   RemovableStorageMap removable_storage_map_;