bind-gen: Implements NVT<IDLNullable<new IDL unions>>

Implements NativeValueTraits for new nullable IDL unions.

Bug: 839389
Change-Id: Ib206f7e1d27c0e0f1c9847bbd7b3188851592586
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2726013
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#858456}

third_party/blink/renderer/bindings/core/v8/native_value_traits_impl.h

@@ -1330,6 +1330,20 @@ struct NativeValueTraits<
   }
 };
 
+template <typename T>
+struct NativeValueTraits<
+    IDLNullable<T>,
+    typename std::enable_if_t<std::is_base_of<bindings::UnionBase, T>::value>>
+    : public NativeValueTraitsBase<T*> {
+  static T* NativeValue(v8::Isolate* isolate,
+                        v8::Local<v8::Value> value,
+                        ExceptionState& exception_state) {
+    if (value->IsNullOrUndefined())
+      return nullptr;
+    return T::Create(isolate, value, exception_state);
+  }
+};
+
 // Migration Adapters: union types generated by the old bindings generator.
 template <typename T>
 struct NativeValueTraits<IDLUnionNotINT<T>> : public NativeValueTraitsBase<T> {