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This is the last major patch to remove the use of direct blob interfaces
from IndexedDB code.

It removes BlobDataHandle from IndexedDBBlobInfo.  There's no longer
any need to load blobs on the IO thread as all the IndexedDBBlobInfo
can be created directly on the idb task runner.

The downside is that tests become a little bit more complicated because
asking for the uuid is asynchronous.

Bug: 1022214
Change-Id: I4bd07cf687b10a0c65efda2044e11bb88171d632
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947388
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721284}