IDB: move LevelDBFactory from singleton to per-bucket instance.

LevelDBFactory holds onto state and configuration for Level DB,
mainly via an instance of LevelDBOptions. LevelDBOptions in turn
points to a number of singletons, as well as holding some POD
options.

Since the important underlying state is still just pointers to
singletons, changing LevelDBFactory use from singleton to various
instances should not have any impact on its own.

However, the existence of the underlying/shared configuration
objects will necessitate great care when attempting to split
level db instances across sequences.

As a result of this CL, IndexedDBClassFactory is finally whittled
down to nothingness.

Bug: 40279485

Change-Id: I8e2ada996fdf119ec7716973e585a3dec936f138
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5299859
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1261896}

• content/browser/BUILD.gn[diff]
• content/browser/indexed_db/indexed_db_backing_store.cc[diff]
• content/browser/indexed_db/indexed_db_backing_store_unittest.cc[diff]
• content/browser/indexed_db/indexed_db_bucket_context.cc[diff]
• content/browser/indexed_db/indexed_db_bucket_context.h[diff]
• content/browser/indexed_db/indexed_db_bucket_context_unittest.cc[diff]
• content/browser/indexed_db/indexed_db_class_factory.cc[Deleted - diff]
• content/browser/indexed_db/indexed_db_class_factory.h[Deleted - diff]
• content/browser/indexed_db/indexed_db_database_unittest.cc[diff]
• content/browser/indexed_db/indexed_db_tombstone_sweeper_unittest.cc[diff]
• content/browser/indexed_db/indexed_db_unittest.cc[diff]