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Per UX request, restoring slight elevation to holding space tray icon
previews to add depth and dilineation.

This CL also makes a slight tweak to shrinking of white backgrounds
that was done to prevent paint artifacts from showing behind images.
Previously we shrunk backgrounds by twice as much as was necessary.

Bug: 1163304
Change-Id: I1b65bb65de3e331f2e58c2993fe1634d5dabcb98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618560Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#841669}