Mark win_xfa_clang as experimental

That way it won't block the CQ while the compile problems are being
investigated.

BUG=chromium:626120
NOTRY=true (otherwise, win_xfa_clang will fail and block this CL)
TBR=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2129893002
diff --git a/infra/config/cq.cfg b/infra/config/cq.cfg
index c22b694..017e68c 100644
--- a/infra/config/cq.cfg
+++ b/infra/config/cq.cfg
@@ -73,6 +73,7 @@
       }
       builders {
         name: "win_xfa_clang"
+        experiment_percentage: 100
       }
       builders {
         name: "win_xfa_64_rel_gn"