Move parallel download feature to components/download

This feature is currently defined in content/.
This Cl moves it to components/download to remove download dependency on content/.

BUG=803135

Change-Id: Iabbc34eff56554722d4aa7bde1f461135984ac74
Reviewed-on: https://chromium-review.googlesource.com/955899


Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542199}