Switch base::Value typemapping to be by value instead of by unique_ptr.

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

 class {{struct.name}}DataView {
  public:
   {{struct.name}}DataView() {}
 
   {{struct.name}}DataView(
       internal::{{struct.name}}_Data* data,
       mojo::internal::SerializationContext* context)
 {%- if struct|requires_context_for_data_view %}
       : data_(data), context_(context) {}
 {%- else %}
@@ skipping 10 matching lines @@
       {{kind|cpp_data_view_type}}* output);
 
   template <typename UserType>
   WARN_UNUSED_RESULT bool Read{{name|under_to_camel}}(UserType* output) {
 {%-     if pf.min_version != 0 %}
     auto* pointer = data_->header_.version >= {{pf.min_version}}
                     ? &data_->{{name}} : nullptr;
 {%-     else %}
     auto* pointer = &data_->{{name}};
 {%-     endif %}
+    if (pointer && pointer->is_null())
+      pointer = nullptr;
+
     return mojo::internal::Deserialize<{{kind|unmapped_type_for_serializer}}>(
         pointer, output, context_);
   }
 
 {%-   elif kind|is_object_kind %}
   inline void Get{{name|under_to_camel}}DataView(
       {{kind|cpp_data_view_type}}* output);
 
   template <typename UserType>
   WARN_UNUSED_RESULT bool Read{{name|under_to_camel}}(UserType* output) {
@@ skipping 68 matching lines @@
 
 {%-   endif %}
 {%- endfor %}
  private:
   internal::{{struct.name}}_Data* data_ = nullptr;
 {%- if struct|requires_context_for_data_view %}
   mojo::internal::SerializationContext* context_ = nullptr;
 {%- endif %}
 };