styleguide: Allow enable_if, conditional, and other type_traits.

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index b6353dc8e568449545d541f2dece3cabd2d55286..4b47bc7978db33f0946a9c30645d4b35f241751e 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -9,6 +9,7 @@
 #define BASE_CALLBACK_INTERNAL_H_
 
 #include <stddef.h>
+#include <type_traits>
 
 #include "base/atomic_ref_count.h"
 #include "base/base_export.h"
@@ -105,18 +106,6 @@ template <typename T> struct IsMoveOnlyType {
                             !is_const<T>::value;
 };
 
-// Returns |Then| as SelectType::Type if |condition| is true. Otherwise returns
-// |Else|.
-template <bool condition, typename Then, typename Else>
-struct SelectType {
-  typedef Then Type;
-};
-
-template <typename Then, typename Else>
-struct SelectType<false, Then, Else> {
-  typedef Else Type;
-};
-
 template <typename>
 struct CallbackParamTraitsForMoveOnlyType;
 
@@ -140,9 +129,9 @@ struct CallbackParamTraitsForNonMoveOnlyType;
 // break passing of C-string literals.
 template <typename T>
 struct CallbackParamTraits
-    : SelectType<IsMoveOnlyType<T>::value,
+    : std::conditional<IsMoveOnlyType<T>::value,
          CallbackParamTraitsForMoveOnlyType<T>,
-         CallbackParamTraitsForNonMoveOnlyType<T> >::Type {
+         CallbackParamTraitsForNonMoveOnlyType<T>>::type {
 };
 
 template <typename T>