Fix potential crash on destruction of SidePanelContentSwappingContainer.

Now that the side panel content is not destroyed when the side panel is
closed it would still exist during destruction. It's possible that when
calling SidePanelContentSwappingContainer::ResetLoadingEntryIfNecessary
during destruction the cached view gets destroyed before retrieving the
content proxy which could cause the crash seen in the bug.

Bug: 339296871
Change-Id: I301516f0e390f4c2e0807c094e77983fd0f42c08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5615270


Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Caroline Rising <corising@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313045}