Add win_clang_x64_dbg to the Windows try bots.

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: "chromium"
 cq_status_url: "https://chromium-cq-status.appspot.com"
 hide_ref_in_committed_msg: true
 commit_burst_delay: 60
 max_commit_burst: 2
 target_ref: "refs/pending/heads/master"
@@ skipping 74 matching lines @@
     buckets {
       name: "tryserver.chromium.win"
       builders {
         name: "win_chromium_compile_dbg_ng_exp"
         experiment_percentage: 10
       }
       builders { name: "win8_chromium_ng" }
       builders { name: "win_chromium_compile_dbg_ng" }
       builders { name: "win_chromium_rel_ng" }
       builders { name: "win_chromium_x64_rel_ng" }
+      builders {
+        name: "win_clang_x64_dbg"
+        experiment_percentage: 5
+      }
     }
   }
 
 # NOTE: when adding new configs to the commit queue, please contact
 # chrome-eng-review per
 # https://sites.google.com/a/chromium.org/dev/developers/testing/chromium-build-infrastructure/tour-of-the-chromium-buildbot?pli=1#TOC-Adding-new-build-configurations-and-tests-to-the-main-Chromium-waterfall-Commit-Queue
 
   sign_cla {}
 }