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This change adds the API method WriteBlobToFile to
blob_storage_context.mojom and adds test for that method. It also adds
some basic tests to the mojo version of BlobStorageContext.

This call tries to optimize the file copy in the case of the blob being
a single file. In that case, it will do a base::CopyFile call, which is
optimized on Windows and should be faster. Otherwise, the API uses the
blob.mojom ReadAll API and stream the data to a file.

R=mek@chromium.org
TBR=ksakamoto@chromium.org

Bug: 1024966, 1036415
Change-Id: I5bf49c39780bd8e25c2ad0f68bb8de7979925284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974700Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731694}