Mojo C++ bindings: expose serialization public API for mojo structs.

mojo/public/interfaces/bindings/tests/test_wtf_types.mojom

Index: mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
diff --git a/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom b/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
index e15e4568f826da86a346e15843ced096c39ad1b4..2fdbea80b6677dca45b24eb1efa3c143badc253e 100644
--- a/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
+++ b/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
@@ -24,6 +24,11 @@ union TestWTFCodeGeneration2 {
   map<string, string?> str_map;
 };
 
+struct TestWTFStruct {
+  string str;
+  int32 integer;
+};
+
 interface TestWTF {
   EchoString(string? str) => (string? str);
   EchoStringArray(array<string?>? arr) => (array<string?>? arr);