Fix crash when long pressing an already closed tab in tab grid search

When an item presented in search results is closed from another window,
the TabSwitcherItem no longer references an exiting WebState. Ensure
long pressing on these items does not cause an application crash. This
CL simply returns an empty list of context menu items which prevents the
context menu from being shown for these tab grid items.

Fixed: 1305243

Change-Id: Ibea0ffebb891646366a234498cd64f3f21d86097
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3514581
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#979798}