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In M89, we introduced a regression where we didn't adequetely protect
the Default Search Provider from removal during keyword deduplication
while the model was still being loaded (loaded_ == false).

This CL fixes that and adds a regression test.

It also adds another CHECK during Remove() which just crashes Chrome if
some other code branch tries to delete the DSE.

Deleting the DSE causes permanent damage to the user's Profile, which
can be synced to other installations of Chrome. It's really bad, so
crashing is not bad in comparison.

Bug: 1164024
Change-Id: If1e70cc2a7b4e0c07b20b4e76acb243d7630937c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623120
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842697}