Add a flag to allow parallel downloading without accept-ranges header in response.

If a download response doesn't have accept-ranges header, we don't fork
parallel requests today. A majority server doesn't send accept-ranges
header even though they support it.
This CL adds a flag to enable parallel downloading even accept-ranges
header are not present.

BUG=971366

Change-Id: I11fc2e7c131a7407d7ee5f269ee5e6b3ff2ce789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648797
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668062}
9 files changed