[CQ] Add coverage bot to cq experiment.

BUG=chromium:568949
LOG=n
TBR=kjellander@chromium.org, tandrii@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1814153002

Cr-Commit-Position: refs/heads/master@{#34882}

infra/config/cq.cfg

@@ -65,13 +65,14 @@ verifiers {
         name: "v8_win_rel_ng_triggered"
         triggered_by: "v8_win_rel_ng"
       }
-    }
-    buckets {
-      name: "tryserver.v8"
       builders {
         name: "v8_linux_blink_rel"
         experiment_percentage: 20
       }
+      builders {
+        name: "v8_linux64_sanitizer_coverage_rel"
+        experiment_percentage: 30
+      }
     }
   }