Harden Widget against null NativeWidget.

This CL ensures that we do not access a NativeWidget that has been
destroyed. This CL also removes |native_widget_destroyed_| and checks
whether the native_widget_ weak ptr is a nullptr instead. The test
WidgetWithDestroyedNativeViewTest is renamed to
WidgetWithDestroyedNativeViewAndNativeWidgetTest since we can now test
for a null NativeWidget.

Bug: 1374015
Change-Id: Ia55cb8af392003688b9977dcd9b26d947f85a92d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4000691
Commit-Queue: Elaine Chien <elainechien@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1068417}