Add presubmit trybot to CQ config.

BUG=470518
TESTED=Ran the trybot on this CL and it seems to work (but fails due to missing LGTM): I'll run it again when I got an LGTM.
R=phoglund@webrtc.org

Review URL: https://codereview.webrtc.org/1188793002.

Cr-Commit-Position: refs/heads/master@{#9450}
diff --git a/infra/config/cq.cfg b/infra/config/cq.cfg
index 2d4a858..8547999 100644
--- a/infra/config/cq.cfg
+++ b/infra/config/cq.cfg
@@ -54,6 +54,7 @@
       builders { name: "mac_x64_gn" }
       builders { name: "mac_x64_gn_rel" }
       builders { name: "mac_x64_rel" }
+      builders { name: "presubmit" }
       builders { name: "win" }
       builders { name: "win_baremetal" }
       builders { name: "win_rel" }