When the device pixel ratio is > 1, image with (device_pixel_ratio *
container_size * max_downscaling_ratio) pixels should be allowed by
max-downscaling-image policy.

BUG:858897

Change-Id: Ic85e79e487c77296556c69b5dc5872e85b22d23e
Reviewed-on: https://chromium-review.googlesource.com/1170853Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Luna Lu <loonybear@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582705}