Add IDBIndexParameters IDL dictionary

Replacing generic dictionary. No changes in behavior. Tests under
storage/indexeddb cover this change. Also cleans up an unnecessary

What the spec defines as IDBObjectStoreParameters dictionary is left
as a generic dictionary for now, as it requires IDL Union support.

Spec: https://dvcs.w3.org/hg/IndexedDB/raw-file/tip/Overview.html#options-object-concept

BUG=403150
R=bashi@chromium.org

Review URL: https://codereview.chromium.org/589663003

git-svn-id: svn://svn.chromium.org/blink/trunk@182526 bbb929c8-8fbe-4397-9dbb-9b2b20218538