Add field-initializing constructors to generated mojo structs.

mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_template_definition.tmpl

 template <typename StructPtrType>
 {{struct.name}}Ptr {{struct.name}}::Clone() const {
-  // Use StructPtrType to prevent the compiler from trying to compile this
-  // without being asked.
-  StructPtrType rv(New());
+  return New(
 {%- for field in struct.fields %}
-  rv->{{field.name}} = mojo::Clone({{field.name}});
+      mojo::Clone({{field.name}})
+{%-   if not loop.last -%},{%- endif %}
 {%- endfor %}
-  return rv;
+  );
 }
 
 template <typename T,
           typename std::enable_if<std::is_same<
               T, {{struct.name}}>::value>::type*>
 bool {{struct.name}}::Equals(const T& other) const {
 {%- for field in struct.fields %}
   if (!mojo::internal::Equals(this->{{field.name}}, other.{{field.name}}))
     return false;
 {%- endfor %}
   return true;
 }