[Wayland] Request XDG activation on window show

For simplicity, a Wayland compositor typically activates a window
when it is mapped. However, it does not necessarily have to and
might not want to in order to prevent stealing focus.

Therefore, explicitly request activation when the window is shown
as active. If it is not configured yet, wait for that to happen.

Bug: 435251395
Change-Id: Idb77506cb8ea0a03b9dd7538904dd5cdbc0c1c75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965756
Commit-Queue: Orko Garai <orko@igalia.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Orko Garai <orko@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1525777}