Mac V2 GPU Sandbox: Turn off crash-on-unknown-denials for now.

Data from the field is showing reduced crashes, so turn it off to
see what the crash rate is without this causing an artificial increase.

Bug: 1126350
Change-Id: I6d2e5722cca8a8d142f510e0a06d926fdc8f202f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2457967
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@{#814870}

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 #t) ; Single-line kill switch.
+(define crash-on-unknown-denials #f) ; Single-line kill switch.
 (if crash-on-unknown-denials
   (begin
     (deny mach-lookup (with no-report)