[cq] Increase commit burst delay.

TBR=jkummerow@chromium.org, hablich@chromium.org
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1258193003

Cr-Commit-Position: refs/heads/master@{#29914}

infra/config/cq.cfg

@@ -5,8 +5,8 @@ version: 1
 cq_name: "v8"
 cq_status_url: "https://chromium-cq-status.appspot.com"
 hide_ref_in_committed_msg: true
-commit_burst_delay: 60
-max_commit_burst: 1
+commit_burst_delay: 300
+max_commit_burst: 3
 target_ref: "refs/pending/heads/master"
 
 rietveld {