Reflect aura::Window's transient parent change in Widget::parent()

An aura::Window may get reparented by
TransientWindowManager::AddTransientChild() without updating
Widget::parent(). This may result in an UAF when Widget::parent() is
dereferenced after destroyed. This CL updates Widget::parent() to
reflect the transient parent change.

Note that aura::Window may also get reparented, but that is not the
widget's logical parent. On ChromeOS, the application windows have a
common native parent, but might have different transient(aka. logical)
parent.

Adds tests to exo_unittests and wm_unittests. The UAF issue seems most
prominent on lacros ChromeOS, where exo provides a
ShellSurface::SetParentWindow() API.

Bug: 1242964
Change-Id: Ie51c7e92ae94e9700aa0c4e158a16d03875aac05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3324727
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#951765}