capture_mode: Make the shelf un-selectable in window capture mode

This CL indirectly fixes an issue where window capture mode on the shelf
would produce a recording with unexpected contents. The underlying
reason was that the window recording would be performed on a descendent
of the shelf rather than the shelf window itself. This CL makes it so
that the shelf will not be selectable for recording.

Users that really want to record the shelf will still have the option to
use the region capture mode.

BUG=b:330124277

Change-Id: Id120e2804493a858d6fe45ca7c1d74726612687f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405086


Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Daniel Andersson <dandersson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280199}