Open Offline Pages in CCT from Downloads Home.

When the respective feature flag is enabled, offline pages opened from
the Downloads Home will use CCT instead of normal tabs.

Bug: 824807
Change-Id: I6d968b8b0c51aaeb7f26332c7ada9f927e151a65
Reviewed-on: https://chromium-review.googlesource.com/977321
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Jian Li <jianli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546545}