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This is a reland of 1bc0f674

Original change's description:
> [IndexedDB] Moving IDBDatabase's PendingConnection to use ScopesLockManager
> 
> This allows scopes to be used in the DeleteDatabase call, and cleans up
> some code in the IndexedDBDatabase class.
> 
> The last part of the codebase that doesn't have locks before being used
> is the IndexedDBBackingStore::Initialize call. Since this happens on the
> opening of the database, this can probably use just the raw database +
> a WriteBatch.
> 
> Bug: 862456
> Change-Id: I89c79ec89a4450e21f1ee700b007ec867b3a4f48
> Reviewed-on: https://chromium-review.googlesource.com/c/1478391
> Commit-Queue: Daniel Murphy <dmurph@chromium.org>
> Reviewed-by: Chase Phillips <cmp@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#634358}

Bug: 862456
Change-Id: I821c28b93fde903678737d49eaeb44c1b7369e08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506552Reviewed-by: Chase Phillips <cmp@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638350}