IndexedDB: Array-type keypaths should not be returned as DOMStringLists

LayoutTests/storage/indexeddb/keypath-arrays-expected.txt

 Test IndexedDB Array-type keyPaths
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 indexedDB = self.indexedDB || self.webkitIndexedDB || self.mozIndexedDB || self.msIndexedDB || self.OIndexedDB;
 
 dbname = "keypath-arrays.html"
 indexedDB.deleteDatabase(dbname)
 indexedDB.open(dbname)
 store = db.createObjectStore('store', {keyPath: ['a', 'b']})
-store.createIndex('index', ['c', 'd'])
+index = store.createIndex('index', ['c', 'd'])
+PASS areArraysEqual(index.keyPath, ['c', 'd']) is true
+PASS index.keyPath instanceof DOMStringList is false
 Expecting exception from db.createObjectStore('store-with-generator', {keyPath: ['a', 'b'], autoIncrement: true})
 PASS Exception was thrown.
 PASS code is DOMException.INVALID_ACCESS_ERR
 Exception message: Failed to execute 'createObjectStore' on 'IDBDatabase': The autoIncrement option was set but the keyPath option was empty or an array.
 Expecting exception from store.createIndex('index-multientry', ['e', 'f'], {multiEntry: true})
 PASS Exception was thrown.
 PASS code is DOMException.INVALID_ACCESS_ERR
 Exception message: Failed to execute 'createIndex' on 'IDBObjectStore': The keyPath argument was an array and the multiEntry option is true.
 
 Empty arrays are not valid key paths:
@@ skipping 18 matching lines @@
 PASS JSON.stringify(cursor.key) is "[1,2]"
 request = index.openCursor()
 cursor = request.result
 PASS cursor is non-null.
 PASS JSON.stringify(cursor.primaryKey) is "[1,2]"
 PASS JSON.stringify(cursor.key) is "[3,4]"
 PASS successfullyParsed is true
 
 TEST COMPLETE