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This definitely looks extremely sketchy when reading code.  Make the
BlobReader owned by a callback itself instead of needing to document
everywhere that a raw new is ok because the class is self-deleting.
Ultimately, this ends up being the same, but is a readability
improvement.

Change-Id: I56f19679a97e3bf220a09fc7f84d3772972b52c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804821
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697716}