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In response to user feedback, a dark gray will be used instead of white
as the tile background.

This is the small merge-safe change. A separate change will remote the
light asset pipeline.

BUG=722573,868343

Change-Id: I4aa7a5a5e2ebba3e9068591dd7b5bfc0d3faa6b5
Reviewed-on: https://chromium-review.googlesource.com/1184308
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584970}