Reenable MacV2GPUSandbox crash-on-unknown-denials.

Also crash on sysctl-read failures.

Bug: 1126350
Change-Id: I9d69c69979ab076e257ca079f6e5f09aed5c28be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493440
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Greg Kerr <kerrnel@chromium.org>
Auto-Submit: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820468}

sandbox/policy/mac/gpu_v2.sb

@@ -16,7 +16,7 @@
 
 ; TODO(https://crbug.com/1126350): Remove this after debugging. These blocks
 ; enumerate known denials, while turning unknown denials into fatal crashes.
-(define crash-on-unknown-denials #f) ; Single-line kill switch.
+(define crash-on-unknown-denials #t) ; Single-line kill switch.
 (if crash-on-unknown-denials
   (begin
     (deny mach-lookup (with no-report)
@@ -49,6 +49,7 @@
       (subpath (param bundle-path))
     )
     (deny file-write* (with send-signal SIGSYS))
+    (deny sysctl-read (with send-signal SIGSYS))
   )
 )