[isolated] Support downloading in client.
[isolated] Support downloading in client.

This change adds support for the 'download' subcommand in the
'isolated' client using the Downloader.

Change-Id: If62fff5fa4f72243b283401aaf756b3fb1efbbcc
Reviewed-on: https://chromium-review.googlesource.com/804394
Commit-Queue: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go