bind-gen: Fix NativeValueTraits<union types>

Adds missing ArgumentValue method to NVT<union types>.

Bug: 839389
Change-Id: I6a8c980eaba4df5ffe4901db6e25abfa6023a174
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2201797Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768773}

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

@@ -1253,6 +1253,13 @@ struct NativeValueTraits<IDLUnionNotINT<T>> : public NativeValueTraitsBase<T> {
                               exception_state);
     return impl;
   }
+
+  static T ArgumentValue(v8::Isolate* isolate,
+                         int argument_index,
+                         v8::Local<v8::Value> value,
+                         ExceptionState& exception_state) {
+    return NativeValue(isolate, value, exception_state);
+  }
 };
 
 template <typename T>
@@ -1266,6 +1273,13 @@ struct NativeValueTraits<IDLUnionINT<T>> : public NativeValueTraitsBase<T> {
                               exception_state);
     return impl;
   }
+
+  static T ArgumentValue(v8::Isolate* isolate,
+                         int argument_index,
+                         v8::Local<v8::Value> value,
+                         ExceptionState& exception_state) {
+    return NativeValue(isolate, value, exception_state);
+  }
 };
 
 template <typename T>
@@ -1279,6 +1293,13 @@ struct NativeValueTraits<IDLNullable<IDLUnionNotINT<T>>>
     return NativeValueTraits<IDLUnionNotINT<T>>::NativeValue(isolate, value,
                                                              exception_state);
   }
+
+  static T ArgumentValue(v8::Isolate* isolate,
+                         int argument_index,
+                         v8::Local<v8::Value> value,
+                         ExceptionState& exception_state) {
+    return NativeValue(isolate, value, exception_state);
+  }
 };
 
 // IDLNullable<IDLUnionINT<T>> must not be used.