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This patch enables multi-threaded LevelDBScopes usage by IndexedDB. It
does this by:
* Changing the mode during scopes initialization,
* Changing the RequestDestruction parameters to accept a WaitableEvent
* Modifies the destructor of IDBOriginState to request this destruction
  & then wait on the event.
* Modifies all unittests to match the new API pattern.

This allows cleanup & revert tasks to now take place off of the main
IndexedDB sequence, ensuring that cleanup & deferred range deletion do
not block IndexedDB operations.

See doc for more reasoning & alternatives:
https://docs.google.com/document/d/1xlSye5iCVU2kRsdzpue3-OE-BIBhmhTfMWss4XYxygI/edit#

Bug: 1058977
Change-Id: I679990a8ef057531a9ad3f710c9411377881eb11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090630Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749233}