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This CL exposes the content layer's fetch_error_body flag in download
service.

Bug: 765778
Change-Id: I2f9ea5b437387e6e1864fb1867436869b6348472
Reviewed-on: https://chromium-review.googlesource.com/707780Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510173}