[Indexeddb] Changed attribute name from multientry to multiEntry

According to the deprecated standard, IndexedDB's Index object should
have attribute named "multientry". The latest standard says that
this attribute should be named "multiEntry" (e is uppercase).

BUG=XWALK-2203
@jizydorczyk thanks for this finding. Do we need to update the 2 test files below per your comment? These 2 files also use this attribute.

- tct-indexeddb-w3c-tests/indexeddb/IDBIndex_name_exist.html
- tct-indexeddb-w3c-tests/indexeddb/IDBIndex_objectStore_exists.html