[CQ] Temporarily reduce coverage bot's frequency

infra/config/cq.cfg

 # See http://luci-config.appspot.com/schemas/projects/refs:cq.cfg for the
 # documentation of this file format.
 
 version: 1
 cq_name: "v8"
 cq_status_url: "https://chromium-cq-status.appspot.com"
 hide_ref_in_committed_msg: true
 commit_burst_delay: 60
 max_commit_burst: 1
 target_ref: "refs/pending/heads/master"
@@ skipping 86 matching lines @@
       builders {
         name: "v8_win_rel_ng_triggered"
         triggered_by: "v8_win_rel_ng"
       }
       builders {
         name: "v8_linux_blink_rel"
         experiment_percentage: 100
       }
       builders {
         name: "v8_linux64_sanitizer_coverage_rel"
-        experiment_percentage: 100
+        experiment_percentage: 20
       }
     }
     buckets {
       name: "master.tryserver.chromium.win"
       builders {
         name: "win_chromium_compile_dbg_ng"
         experiment_percentage: 100
       }
     }
   }
 
   sign_cla {}
 }