cq: explicitly opt out of tryjob cancelation.

This is to ensure that CQ's default can be switched to "cancel" for all
other projects w/o affecting (negatively or positively) chromium.

R=martiniss

Bug: 959436
Change-Id: I25be0b99ad179a3847c141056beb295ec1c7ca88
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1791901
Auto-Submit: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694810}

infra/config/commit-queue.cfg

@@ -50,6 +50,8 @@ config_groups {
       url: "https://chromium-status.appspot.com/"
     }
     tryjob {
+      # TODO(crbug/959436): enable it.
+      cancel_stale_tryjobs: NO
       #############################
       # Always required builders. #
       #############################
@@ -409,6 +411,8 @@ config_groups {
       url: "https://chromium-status.appspot.com/"
     }
     tryjob {
+      # TODO(crbug/959436): enable it.
+      cancel_stale_tryjobs: NO
       ###########################################
       # Experimental builders for branch-heads. #
       ###########################################