[Shelf] Speculative fixing for ShelfView::PrepareForDrag

Have not found a way to reproduce the issue on a real device yet. But
by reading code, I noticed that the data members for the in-flight
context menu model request are not reset properly when a shelf item
is removed.

This CL fixes this issue by resetting `context_menu_callback_` and
`item_awaiting_response_` in `ShelfView::PrepareForDrag()` if the
removed item has an active request for context menu model.

Add a unit test to verify this scenario.

Bug: 1322950
Change-Id: I6b9149ea9eb1bdb91b052b9989b1d485811b1d1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3639442
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1001712}