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

mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h

Index: mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h
diff --git a/mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h b/mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h
index 6b3acf69b4c385648037a95d7ffee51c13bae944..8f9e78ec032413024c252d0f7880f69a23b05d74 100644
--- a/mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h
+++ b/mojo/public/cpp/bindings/tests/struct_with_traits_impl_traits.h
@@ -20,11 +20,15 @@ namespace mojo {
 template <>
 struct StructTraits<test::NestedStructWithTraits,
                     test::NestedStructWithTraitsImpl> {
+  static void* SetUpContext(const test::NestedStructWithTraitsImpl& input);
+  static void TearDownContext(const test::NestedStructWithTraitsImpl& input,
+                              void* context);
+
+  static int32_t value(const test::NestedStructWithTraitsImpl& input,
+                       void* context);
+
   static bool Read(test::NestedStructWithTraitsDataView data,
                    test::NestedStructWithTraitsImpl* output);
-  static int32_t value(const test::NestedStructWithTraitsImpl& input) {
-    return input.value;
-  }
 };
 
 template <>