Add TypeTraits template for checking safe bitfields (WIP)

third_party/WebKit/Source/wtf/TypeTraits.h

Index: third_party/WebKit/Source/wtf/TypeTraits.h
diff --git a/third_party/WebKit/Source/wtf/TypeTraits.h b/third_party/WebKit/Source/wtf/TypeTraits.h
index c5199c173c2c4d6b9286d2ee525480ff9e326986..f0a0e7d6b167550370a26e2be94b5d2cfa100621 100644
--- a/third_party/WebKit/Source/wtf/TypeTraits.h
+++ b/third_party/WebKit/Source/wtf/TypeTraits.h
@@ -23,9 +23,11 @@
 #define TypeTraits_h
 
 #include <cstddef>
+#include <limits>
 #include <type_traits>
 #include <utility>
 
+#include "base/template_util.h"
 #include "wtf/Compiler.h"
 
 namespace WTF {
@@ -141,6 +143,18 @@ template <typename T> struct IsTriviallyDestructible {
 #endif
 };
 
+// In MSVC, enums default to 'int' as their underlying type, which means when
+// they are interpreted as signed when read out of a bitfield. This causes
+// issues when trying to serialize/deserialize an enum from a bitfield. To fix
+// this, ensure that all bitfields use an unsigned underlying type, e.g.
+// enum class Foo : unsigned {}.
+// TODO(sashab): Migrate this to be a clang plugin instead.
+template <typename T>
+struct IsEnumSafeToStoreInBitfield {
+    static_assert(std::is_enum<T>::value, "T must be an enum.");
+    static const bool value = !std::numeric_limits<base::underlying_type<T>>::is_signed;
+};
+
 template <typename T, typename U> struct IsSubclass {
 private:
     typedef char YesType;