IDB: get rid of IndexedDBValue copy ctor.

This copy constructor is a footgun. There's one place in production code
where it was used mistakenly, which is updated. There's another place in
production code where it was used intentionally, and that's replaced
with an explicit Clone() method.

Bug: none
Change-Id: I10c08a659f0d741af3191637918674a3ecd1af0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6507450
Commit-Queue: Evan Stade <evanstade@microsoft.com>
Reviewed-by: Steve Becker <stevebe@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1457080}