Add Install Google Drive button tap handler.

When the button is tapped, DownloadManagerCoordinator does the following:
 - starts StoreKitCoordinator (which presents StoreKit dialog)
 - hides Install Google Drive Button

UI Mocks: https://docs.google.com/presentation/d/1GzbAoJrpW9IAQF78afh5SZLWJWErNcC67t_ctujjEus/edit#slide=id.g2eee39b982_0_0

Bug: 791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I2bcc400e9fafbde54f251fc05e8b607cc70e463b
Reviewed-on: https://chromium-review.googlesource.com/937995
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539570}
5 files changed