Adds side-shadow assets to the Overlay in the new layout that already
has rounded corners.  These assets just add a shadow, so there are
no dark versions needed, and we just have one side (using code to
flip it for the other side).

Binary-Size: +2K for the side shadow assets.

BUG=1005975

Change-Id: I31ebf657b2e9f49822c5aeb525b655c91d9b81aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895924
Commit-Queue: Donn Denman <donnd@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Auto-Submit: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713660}