[lensoverlay] Add corner radius

Adds a corner radius appropriately for the side panel along with
the bottom corners which are only rounded on Mac.

Note: Ideally there would be a way to clip our overlay to always
be without what is rendered to the screen of the WebContents
underneath, but I couldn't find an easy way to do this. So for now,
handling on a case by case basis.

Bug: b:339019931
Change-Id: I01f3e13efa5424920a06cf2f4b6efa93bf6a878f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5536101
Commit-Queue: Duncan Mercer <mercerd@google.com>
Reviewed-by: Juan Mojica <juanmojica@google.com>
Cr-Commit-Position: refs/heads/main@{#1300686}