IndexedDB: add deserialization code for sortable IDB keys

Bug: 1409618
Change-Id: I43fe51e77c1da43e707430cacaf8debdeacb7a65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4651525


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1164358}