Ensure that top-level windows show up in Mission Control

When NativeWidgetNSWindowBridge::SetParent() made a window be a child
window, it marked it as having transient collection behavior.

However, when it determined that the window should be a top-level
window, it didn’t correctly strip that bit.

Therefore, ensure that when a window is made to be a top-level window,
it regains the expected managed collection behavior.

Fixed: 396691147
Include-Ci-Only-Tests: true
Change-Id: I6a3ed381c2dc5e7cd1491bbcd6b240970abddf94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6293385
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423653}