Mojo C++ bindings: support attaching context object for StructTraits.

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set mojom_type = struct|get_qualified_name_for_kind %}
 {%- set data_type = struct|get_qualified_name_for_kind(internal=True) %}
 
 template <>
 struct StructTraits<{{mojom_type}}, {{mojom_type}}Ptr> {
   static bool IsNull(const {{mojom_type}}Ptr& input) { return !input; }
   static void SetToNull({{mojom_type}}Ptr* output) { output->reset(); }
 
 {%- for field in struct.fields %}
 {%-   set return_ref = field.kind|is_object_kind or
                        field.kind|is_any_handle_kind or
                        field.kind|is_interface_kind or
                        field.kind|is_associated_kind %}
 {%-   if return_ref %}
   static decltype({{mojom_type}}::{{field.name}})& {{field.name}}(
       {{mojom_type}}Ptr& input) {
     return input->{{field.name}};
   }
-  static const decltype({{mojom_type}}::{{field.name}})& {{field.name}}(
-      const {{mojom_type}}Ptr& input) {
-    return input->{{field.name}};
-  }
 {%-   else %}
   static decltype({{mojom_type}}::{{field.name}}) {{field.name}}(
       const {{mojom_type}}Ptr& input) {
     return input->{{field.name}};
   }
 {%-   endif %}
 {%- endfor %}
 
   static bool Read({{mojom_type}}DataView input, {{mojom_type}}Ptr* output);
 };
 
 namespace internal {
 
 template <typename MaybeConstUserType>
 struct Serializer<{{mojom_type}}Ptr, MaybeConstUserType> {
   using UserType = typename std::remove_const<MaybeConstUserType>::type;
   using Traits = StructTraits<{{mojom_type}}, UserType>;
 
   static size_t PrepareToSerialize(MaybeConstUserType& input,
                                    SerializationContext* context) {
     if (CallIsNullIfExists<Traits>(input))
       return 0;
-    {{struct_macros.get_serialized_size(struct, "Traits::%s(input)", "context",
-                                        True)|indent(2)}}
+
+    void* custom_context = CustomContextHelper<Traits>::SetUp(input, context);
+    ALLOW_UNUSED_LOCAL(custom_context);
+
+    {{struct_macros.get_serialized_size(
+          struct, "CallWithContext(Traits::%s, input, custom_context)",
+          "context", True)|indent(2)}}
     return size;
   }
 
   static void Serialize(MaybeConstUserType& input,
                         Buffer* buffer,
                         {{data_type}}** output,
                         SerializationContext* context) {
-    if (!CallIsNullIfExists<Traits>(input)) {
-      {{struct_macros.serialize(struct, struct.name ~ " struct",
-                                "Traits::%s(input)", "result", "buffer",
-                                "context", True)|indent(4)}}
-      *output = result;
-    } else {
+    if (CallIsNullIfExists<Traits>(input)) {
       *output = nullptr;
+      return;
     }
+
+    void* custom_context = CustomContextHelper<Traits>::GetNext(context);
+
+    {{struct_macros.serialize(
+          struct, struct.name ~ " struct",
+          "CallWithContext(Traits::%s, input, custom_context)", "result",
+          "buffer", "context", True)|indent(4)}}
+    *output = result;
+
+    CustomContextHelper<Traits>::TearDown(input, custom_context);
   }
 
   static bool Deserialize({{data_type}}* input,
                           UserType* output,
                           SerializationContext* context) {
     if (!input)
       return CallSetToNullIfExists<Traits>(output);
 
     {{mojom_type}}DataView data_view(input, context);
     return Traits::Read(data_view, output);
   }
 };
 
 }  // namespace internal