Fix PRESUBMIT.py after disabling CQ.

The disabling of CQ in https://codereview.webrtc.org/1177243003/
broke PRESUBMIT.py since it's trying to read cq.cfg.

BUG=501332
TBR=phoglund@webrtc.org

Review URL: https://codereview.webrtc.org/1192673003.

Cr-Commit-Position: refs/heads/master@{#9462}