Fix x11::WindowCache::GetWindowAtPoint() in i3

In i3, container windows have a WM_NAME, but we're interested in the
named content window.  This CL changes the DFS search order to check
child windows before checking parent windows.

R=sky

Fixed: 1316735
Change-Id: I8049552f1c0faa7dcbc3bb6b25df2324ee9bbf7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3588704
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#993220}