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Instead just read the blob through the mojom Blob interface directly.
For now still relying on blob UUIDs, but ultimately code calling into
this should already have a BlobPtr rather than having to ever look it
up from a UUID.

Bug: 701851
Change-Id: If267c750368cc4a43d52d239b8462cc65d7214bb
Reviewed-on: https://chromium-review.googlesource.com/1101899Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568063}