Use NOTREACHED_IN_MIGRATION() in c/b/extensions/

This was generated by replacing "  NOTREACHED()" with
"  NOTREACHED_IN_MIGRATION()" and running git cl format.

This prepares for making NOTREACHED() [[noreturn]] alongside
NotReachedIsFatal migration of existing inventory.

Bug: 40580068
Change-Id: I1ba872fd8f6d8ef17b651362d0ddab624dc80e8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5539173


Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Owners-Override: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300854}