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This CL introduces a backend that uses URLFetcher and save in memory
download data to a blob.

Currently there is an issue that only content/browser uses
storage::BlobStorageContext and content/public does not expose it.
Only BrowserContext::CreateMemoryBackedBlob does the browser side save
blob task for media API, which only expose the UUID in
content::BlobHandle.

Bug: 95809
Change-Id: Iac93952dd9a73a8c3eba6e6a1d1bf642a9a0d50f
Reviewed-on: https://chromium-review.googlesource.com/833026
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526733}