Safe Browsing: clean up the BillingInterstitial Feature

This removes the kBillingInterstitial base::Feature, since we want this
functionality to be enabled-by-default.

This also cleans up the fieldtrial config.

Bug: 933807
Test: None
Change-Id: If03b3bba5a3abdb375b7f83fd9d76171df641763
Reviewed-on: https://chromium-review.googlesource.com/c/1479277
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634078}
9 files changed