Fix destroyed web contents pointer crash in BubbleSignInPromoDelegate

When a signin/sync promo view is opened, the current web contents are
attached to a delegate in order to retrieve the current profile and
potentially open a sign in tab if the user accepts the bubble. If the
web contents are destroyed while the bubble is opened, and the bubble
does not close, the pointer becomes invalid. This caused a crash when
trying to retrieve the profile from the web content's browser context.
In this case we should do nothing instead, the bubble will close by
itself.

Bug: 411322191
Change-Id: I0ae67c9709393be003043310b94c5992e80066e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6469696
Commit-Queue: Amelie Schneider <amelies@google.com>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1448876}