Allow capturing editor window if slideshow is chosen for capturing

This change ties the capture session to the editor window when a slide
show is chosen for capture. This ensures that the present session
continues by capturing the editor window in the case when the user has
exited the slide show.

Bug: chromium:409473386
Change-Id: I8c141f80c193d53acb3a763b4c6d5976034d607b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/407423
Commit-Queue: Palak Agarwal <agpalak@google.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46634}