Mojo C++ bindings: more consistent Clone() and Equals().

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

Index: mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
diff --git a/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom b/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
index 8763540361008ea9a43397fa99d4f099003ae681..11097c04934f150e23e5007338f677ea4edeb0ba 100644
--- a/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
+++ b/mojo/public/interfaces/bindings/tests/struct_with_traits.mojom
@@ -20,6 +20,7 @@ struct StructWithTraits {
   map<string, NestedStructWithTraits> f_struct_map;
 };
 
+// Test that this container can be cloned.
 struct StructWithTraitsContainer {
   StructWithTraits f_struct;
 };
@@ -28,6 +29,12 @@ struct PassByValueStructWithTraits {
   handle f_handle;
 };
 
+// The custom type for PassByValueStructWithTraits is not clonable. Test that
+// this container can compile as long as Clone() is not used.
+struct PassByValueStructWithTraitsContainer {
+  PassByValueStructWithTraits f_struct;
+};
+
 interface TraitsTestService {
   EchoStructWithTraits(StructWithTraits s) => (StructWithTraits passed);