GitLab

This method will be useful for other systems that want to handle
download themselves, especially offline pages, which can intercept
the download here onwards based on mime_type.

Bug: 802309
Change-Id: I5b7bfd7840334979d082c6afa983668ceb020968
Reviewed-on: https://chromium-review.googlesource.com/879548
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537008}