bindings: Make the sequence conversion code more complaint with WebIDL.

third_party/WebKit/Source/core/testing/SequenceTest.idl

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+enum FoodEnum {
+    "Bread",
+    "Spaghetti",
+    "Sushi"
+};
+
+interface SequenceTest {
+    // The identity functions just return what they have received in |arg|, to
+    // test converting both to and from V8.
+    sequence<sequence<ByteString>> identityByteStringSequenceSequence(sequence<sequence<ByteString>> arg);
+    sequence<double> identityDoubleSequence(sequence<double> arg);
+    sequence<FoodEnum> identityFoodEnumSequence(sequence<FoodEnum> arg);
+    sequence<long> identityLongSequence(sequence<long> arg);
+    sequence<octet>? identityOctetSequenceOrNull(sequence<octet>? arg);
+
+    sequence<Element> getElementSequence();
+    void setElementSequence(sequence<Element> arg);
+
+    bool unionReceivedSequence((double or sequence<double>) arg);
+};