Bump execution timeout of mac libfuzzer bot from 3hr to 4hr.

A full build lasts a bit longer than 3hr:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Libfuzzer%20Upload%20Mac%20ASan

Bug: 903591
Change-Id: Iac092514350db2381ad3596b48fbd487522e0789
Reviewed-on: https://chromium-review.googlesource.com/c/1384749
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617908}

infra/config/global/cr-buildbucket.cfg

@@ -2026,6 +2026,7 @@ buckets {
       dimensions: "cores:24"
       mixins: "fuzz-ci"
       mixins: "libfuzzer"
+      execution_timeout_secs: 14400 # 4 hours
     }
     builders {
       name: "Libfuzzer Upload Windows ASan"