Drop FocusManager pointer in NativeWidgetMac during Close().

Context:
https://chromium-review.googlesource.com/c/chromium/src/+/5598058/comment/542e980a_676c4536/

NativeWidgetMac holds a raw_ptr to the Widget's FocusManager. If
a client instantiates a Widget with the CLIENT_OWNS_WIDGET ownership
model, the Widget can be destroyed first (FocusManager is also
destroyed) while NativeWidgetMac lives on for a little longer
before it's destroyed. The raw_ptr to the FocusManager inside
NativeWidgetMac is not getting set to null when the Widget is
being deleted, resulting in a dangling raw_ptr failure. This also
results in a use-after since the NativeWidgetMac tries to unregister
itself as a FocusChangeListener after FocusManager has already been
destroyed.

A test case that exercises this code path will be added in the end
when all fixes to make it pass have been landed.

Bug: b:294094124
Change-Id: I71b793959681ed12bd01a4bdebe8a3b9b39242a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617579
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Eric Sum <esum@google.com>
Cr-Commit-Position: refs/heads/main@{#1313762}