Double Mac FYI Experimental Retina Release (NVIDIA) timeout, to 12h.

TBR=kbr@chromium.org

Bug: 853307
Change-Id: Id7ec4011b055a592f5a6499b1495425acd64474d
Reviewed-on: https://chromium-review.googlesource.com/1105512Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568287}

infra/config/global/cr-buildbucket.cfg

@@ -1204,7 +1204,10 @@ buckets {
     builders {
       name: "Mac FYI Experimental Retina Release (NVIDIA)"
       mixins: "mac-gpu-fyi-ci"
-      mixins: "gpu-slow-bot"
+      # This bot has one machine backing its tests at the moment.
+      # If it gets more, this should be switched back to gpu-slow-fyi.
+      # See crbug.com/853307 for more context.
+      execution_timeout_secs: 43200
     }
     builders {
       name: "Mac FYI GPU ASAN Release"