[autofill][desktop][bugfix] Remove the explanation view during loading

When showing the Save Card loading throbber, if the extra view is set,
reset the extra view back to nullptr.

Before, DialogClientView would not unset the extra view when nullptr
was set on DialogDelegate::SetExtraView. Now, update the
DialogClientView::UpdateExtraViewFromDelegate() so if the new extra view
is nullptr, remove the existing extra view if there is one. Change
DialogDelegate::DisownExtraView() to return an optional type value so
when the extra view hasn't been explicitly set, nullopt is returned
indicating that the extra view should not be updated..

Bug: 329678158
Change-Id: I2b8e867af2e152b46fea911374f1a46e70011089
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5384657


Reviewed-by: Slobodan Pejic <slobodan@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Darwin Yang <darwinyang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277164}