IndexedDB: add flag for changing default IDB transaction durability

Note that this only affects readwrite transactions created via
`IDBDatabase.transaction`. The following are NOT affected:

* For "read" transactions, the flush param is ignored;
  see `LevelDBScope::Commit`
* The transaction for a version change
  operation is always strict; see `OpenRequest::StartUpgrade`.
* The transaction for deleting a database is always relaxed;
  see `IndexedDBBackingStore::DeleteDatabase`
* Various internal operations like database upgrades or
  initialization use LevelDBDirectTransaction/
  TransactionalLevelDBDatabase which hard codes syncing to
  true (see kSyncWrites)

Bug: 965883
Change-Id: Ida072dc6cc3908a37a06277c7926d267c9969ca2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4898221
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1207923}