Move checking of previews blacklist flag to params

This enables other flags (in the next CL) to enable this behavior.

Bug: 956685
Change-Id: I94e909bda16ff52ad9f5f794ade14dafbc198417
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619214
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661366}
8 files changed