[ios] Remove rootViewController from /download

A method (-controllerForUnhandledContent:) was in CRWWebDelegate when
it belongs in CRWNativeContentProvider protocol.
This helped pull the implementation out of tab.mm and into the BVC,
which can provide a base view controller for presenting UI,
instead of using rootViewController for presentation.

Bug: 791206
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: If82dcb4f16a64d5e1fef99e2805588e92f2ca4d9
Reviewed-on: https://chromium-review.googlesource.com/807306
Commit-Queue: edchin <[email protected]>
Reviewed-by: Mark Cogan <[email protected]>
Reviewed-by: edchin <[email protected]>
Reviewed-by: Eugene But <[email protected]>
Cr-Commit-Position: refs/heads/master@{#521591}
12 files changed