Move casts.h and stl_util.h into base/.

Change-Id: Idb3bfa5cec3d146e488031d91265737cde2b4cf4
diff --git a/src/atomic.cc b/src/atomic.cc
index 5cc750d..480e456 100644
--- a/src/atomic.cc
+++ b/src/atomic.cc
@@ -20,7 +20,7 @@
 #include <vector>
 
 #include "base/mutex.h"
-#include "stl_util.h"
+#include "base/stl_util.h"
 #include "stringprintf.h"
 #include "thread.h"