Mark builders with Win10 GCE slaves as experimental.

Bug: 819355
Change-Id: Ib242933a5a3d1cf4d03edac0cb4b4e6be468b12d
Reviewed-on: https://chromium-review.googlesource.com/952777Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: smut <smut@google.com>
Cr-Commit-Position: refs/heads/master@{#541326}

infra/config/branch/cq.cfg

@@ -156,11 +156,15 @@ verifiers {
       }
       builders {
         name: "win7_chromium_rel_ng"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        # TODO(jchinlee): uncomment when crbug/819355 is fixed.
+        #equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        experiment_percentage: 100
       }
       builders {
         name: "win10_chromium_x64_rel_ng"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        # TODO(jchinlee): uncomment when crbug/819355 is fixed.
+        #equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        experiment_percentage: 100
       }
       builders {
         name: "win_chromium_compile_dbg_ng"