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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}