[lensoverlay] Add shadow to preselection bubble on Windows

A recent change outside of the Chrome binary caused a rendering bug
in the coachmark bubble. A long term solution was submitted in
crerev.com/c/6337512, but that is not a safe cherry pick. Since this
is broken in Stable Chrome on Windows, this CL is a band aid fix to
that is safer to cherrypick as far back as possible.

Bug: 399737950
Change-Id: I9e8242d2fdb003b206b4744884672bb539a73584
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6339968
Reviewed-by: Juan Mojica <juanmojica@google.com>
Commit-Queue: Duncan Mercer <mercerd@google.com>
Cr-Commit-Position: refs/heads/main@{#1430396}