Add a flag to separately enable hover card preview images.

CL which actually implements images is forthcoming.

Bug: 928954
Change-Id: I55e829aed86c8a5d8a7ad3e644bd722216a336d1
Reviewed-on: https://chromium-review.googlesource.com/c/1455656
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630132}
7 files changed