Mark DOMStringList.contains() as deprecated, add console warning.

LayoutTests/storage/indexeddb/mozilla/resources/create-index-unique.js

 // original test:
 // http://mxr.mozilla.org/mozilla2.0/source/dom/indexedDB/test/test_create_index.html
 // license of original test:
 // " Any copyright is dedicated to the Public Domain.
 //   http://creativecommons.org/publicdomain/zero/1.0/ "
 
 if (this.importScripts) {
     importScripts('../../../../resources/js-test.js');
     importScripts('../../resources/shared.js');
 }
@@ skipping 10 matching lines @@
     indexName = "1";
     indexKeyPath = "unique_value";
     index = evalAndLog("index = objectStore.createIndex(indexName, indexKeyPath, { unique: true });");
     shouldBe("index.name", "indexName");
     shouldBe("index.keyPath", "indexKeyPath");
     shouldBe("index.unique", "true");
     shouldBe("objectStore.indexNames.length", "1");
 
     foundNewlyCreatedIndex = false;
     for (var k = 0; k < objectStore.indexNames.length; k++) {
-        if (objectStore.indexNames.item(k) == indexName) {
+        if (objectStore.indexNames[k] == indexName) {
             foundNewlyCreatedIndex = true;
         }
     }
     shouldBeTrue("foundNewlyCreatedIndex");
     shouldBe("event.target.transaction.db", "db");
     shouldBe("event.target.transaction.mode", "'versionchange'");
     finishJSTest();
 }