Add OVERRIDE to base/.

base/threading/thread_collision_warner.h

Index: base/threading/thread_collision_warner.h
diff --git a/base/threading/thread_collision_warner.h b/base/threading/thread_collision_warner.h
index 24f09205886285051055c5e957bfa9f52689a73f..a9dfc0a4cfb2a71bd87f406fff3be0de87a02640 100644
--- a/base/threading/thread_collision_warner.h
+++ b/base/threading/thread_collision_warner.h
@@ -8,8 +8,9 @@
 
 #include <memory>
 
-#include "base/base_export.h"
 #include "base/atomicops.h"
+#include "base/base_export.h"
+#include "base/compiler_specific.h"
 
 // A helper class alongside macros to be used to verify assumptions about thread
 // safety of a class.
@@ -138,7 +139,7 @@ struct BASE_EXPORT AsserterBase {
 
 struct BASE_EXPORT DCheckAsserter : public AsserterBase {
   virtual ~DCheckAsserter() {}
-  virtual void warn();
+  virtual void warn() OVERRIDE;
 };
 
 class BASE_EXPORT ThreadCollisionWarner {