chromium.memory - Mac ASan 64 builder: allow 2 concurrent builds

Theres's a working theory that it's just a bad bot.  run 2 bots concurrently and check results.

Bug: 855396
Change-Id: I44da7322a5302b7d571eff3eb768f926c5cad9c7
Reviewed-on: https://chromium-review.googlesource.com/1112065Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569759}

infra/config/global/luci-scheduler.cfg

@@ -2935,6 +2935,9 @@ job {
     bucket: "luci.chromium.ci"
     builder: "Mac ASan 64 Builder"
   }
+  triggering_policy: {
+    max_concurrent_invocations: 2
+  }
 }
 
 job {