Mojo C++ bindings: introduce mojo::WTFMap for blink bindings.

mojo/public/cpp/bindings/lib/map_serialization.h

Index: mojo/public/cpp/bindings/lib/map_serialization.h
diff --git a/mojo/public/cpp/bindings/lib/map_serialization.h b/mojo/public/cpp/bindings/lib/map_serialization.h
index 91e404e478abe9518e6790da88073fa4c70cc4c1..e7dc2b0681364ea84166686ca38342f4c7a16db8 100644
--- a/mojo/public/cpp/bindings/lib/map_serialization.h
+++ b/mojo/public/cpp/bindings/lib/map_serialization.h
@@ -6,6 +6,7 @@
 #define MOJO_PUBLIC_CPP_BINDINGS_LIB_MAP_SERIALIZATION_H_
 
 #include <type_traits>
+#include <vector>
 
 #include "mojo/public/cpp/bindings/array.h"
 #include "mojo/public/cpp/bindings/lib/array_serialization.h"
@@ -84,11 +85,11 @@ struct Serializer<Map<Key, Value>, MaybeConstUserType> {
   using UserValue = typename Traits::Value;
   using Data = typename Map<Key, Value>::Data_;
   using KeyArraySerializer = ArraySerializer<Array<Key>,
-                                             Array<UserKey>,
+                                             std::vector<UserKey>,
                                              MapKeyReader<MaybeConstUserType>>;
   using ValueArraySerializer =
       ArraySerializer<Array<Value>,
-                      Array<UserValue>,
+                      std::vector<UserValue>,
                       MapValueReader<MaybeConstUserType>>;
 
   static size_t PrepareToSerialize(MaybeConstUserType& input,
@@ -150,8 +151,8 @@ struct Serializer<Map<Key, Value>, MaybeConstUserType> {
     if (!input)
       return CallSetToNullIfExists<Traits>(output);
 
-    Array<UserKey> keys;
-    Array<UserValue> values;
+    std::vector<UserKey> keys;
+    std::vector<UserValue> values;
 
     if (!KeyArraySerializer::DeserializeElements(input->keys.ptr, &keys,
                                                  context) ||