capture_game: Bar anchoring if the selected window bounds change

Fixed: b/283120954
Change-Id: If6ec7ff6e066c3e8ad8fc4b7e6348f333f8c2c4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4568604


Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149442}