After research, tentatively updating the unoptimized-images policy as:
no using more than 0.5 byte per pixel of image data with approximate
header size (1KB) removed.

Bug: 901925
Change-Id: Ibc5b2e2e6ccaba7a273c63440d48d4e0d154cc2e
Reviewed-on: https://chromium-review.googlesource.com/c/1318372Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Paul Meyer <paulmeyer@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605677}