[Ozone/Wayland]Rename child_window to child_popup

To clearly differentiate it from child_bubbles.

Change-Id: I9be0d4fac3452fbbec857444aad080c6d8ba0651
Bug: 329705709
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5535619


Commit-Queue: Kramer Ge <fangzhoug@chromium.org>
Reviewed-by: Peter McNeeley <petermcneeley@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300869}