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In http://crbug.com/957175, wanderview@ added an optimization for
BlobReader where it would add an extra hop when finishing a stream.
Many blobs size mojo::DataPipe to exactly fit the blob size, and so
this can be quite common.

As SourceStreamToDataPipe is going to be used in this CL
(https://chromium-review.googlesource.com/c/chromium/src/+/1830360),
that optimization needs to be carried forward.

Bug: 1012869,957175
Change-Id: I9f7fc006b633a248fe3b726ac78d1fe4773472a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862253Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707920}