IndexedDB: add deserialization code for sortable IDB keys

The first form of this change was landed as
32257e5b but it was reverted
due to msan failures. Since then, the underlying code has
changed a bit to use base::numerics:: endian conversions, so
the new patch is not quite identical to the old one.

Bug: 1409618
Change-Id: I993312821d9ca459a3ce936c38a9ce1758e13157
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5403298


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280208}