Use NOTREACHED() after //sandbox switch statements

Change switch statements that exhaustively handle all cases to have an
explicit NOTREACHED() in the unreachable part where the switch
statements end. Per https://abseil.io/tips/147, this helps prevent the
compiler from generating code with undefine behavior in some situations.

Change-Id: Ie2cfbeec1527da9d72ecd887b1936033ef567e3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6696799
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1488648}