[Blobs] Refactor extensions BlobReader to no longer use blob URLs.

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/1101899
Reviewed-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}
8 files changed