Add a default clause to the switch statement in debug::GetBuiltin() and
abort if it is reached.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I8895497b64ccd3b8eb23d281b8bca4405a5c3959
Reviewed-on: https://chromium-review.googlesource.com/574172Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Ben Noordhuis <info@bnoordhuis.nl>
Cr-Commit-Position: refs/heads/master@{#46712}