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

mojo/public/cpp/bindings/wtf_array.h

Index: mojo/public/cpp/bindings/wtf_array.h
diff --git a/mojo/public/cpp/bindings/wtf_array.h b/mojo/public/cpp/bindings/wtf_array.h
index 0e157f2ceea1788bcb1e846dcf99b39ee0f8646b..45014470741b691d964bbe2777b224a4a094e85b 100644
--- a/mojo/public/cpp/bindings/wtf_array.h
+++ b/mojo/public/cpp/bindings/wtf_array.h
@@ -28,10 +28,6 @@ class WTFArray {
   MOVE_ONLY_TYPE_FOR_CPP_03(WTFArray);
 
  public:
-  using Data_ = internal::Array_Data<
-      typename internal::GetDataTypeAsArrayElement<T>::Data>;
-  using Element = T;
-
   // Constructs an empty array.
   WTFArray() : is_null_(false) {}
   // Constructs a null array.