Convert indexed_db_leveldb_* to StorageKey from url::Origin

This is the first of a series of CLs to convert IndexedDB files to use
the new StorageKey class. Starting from the bottom and pushing up to
the outer edge.

Bug: 1210555
Change-Id: Ie36a11b8056a7e57e39ff21702856f30ac45f37b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2915222


Commit-Queue: Ari Chivukula <arichiv@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Steven Bingler <bingler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#887354}