Mojo C++ bindings: change the first template parameter of StructTraits and UnionTraits.

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

 {%- set mojom_type = union|get_qualified_name_for_kind %}
 
 // static
-bool UnionTraits<{{mojom_type}}, {{mojom_type}}Ptr>::Read(
-    {{mojom_type}}DataView input,
+bool UnionTraits<{{mojom_type}}::DataView, {{mojom_type}}Ptr>::Read(
+    {{mojom_type}}::DataView input,
     {{mojom_type}}Ptr* output) {
   *output = {{mojom_type}}::New();
   {{mojom_type}}Ptr& result = *output;
 
   internal::UnionAccessor<{{mojom_type}}> result_acc(result.get());
   switch (input.tag()) {
 {%- for field in union.fields %}
     case {{mojom_type}}::Tag::{{field.name|upper}}: {
 {%-   set name = field.name %}
 {%-   set kind = field.kind %}
@@ skipping 17 matching lines @@
       result->set_{{name}}(input.{{name}}());
 {%-   endif %}
       break;
     }
 {%- endfor %}
     default:
       return false;
   }
   return true;
 }