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}
1 file changed