Add linux_memcheck as default trybot.

After moving to Swarming, this bot executes in ~20 minutes
instead of hours, making it a good candidate for pre-submit
checks. It would have caught the leak that led to the revert
in https://codereview.webrtc.org/2649113003/ if it would have run.

BUG=chromium:497757
TBR=ehmaldonado@webrtc.org

Review-Url: https://codereview.webrtc.org/2649353002 .
Cr-Commit-Position: refs/heads/master@{#16226}

infra/config/cq.cfg

@@ -48,6 +48,7 @@ verifiers {
       builders { name: "linux_baremetal" }
       builders { name: "linux_compile_dbg" }
       builders { name: "linux_libfuzzer_rel" }
+      builders { name: "linux_memcheck" }
       builders { name: "linux_msan" }
       builders { name: "linux_rel" }
       builders { name: "linux_tsan2" }