[tools] Reduce experimental load on Mac to avoid too many pending builds

R=machenbach@chromium.org

Bug: chromium:747960
Change-Id: Id4e3b976e2600b075a3ef5b719490a9d749b3760
Reviewed-on: https://chromium-review.googlesource.com/813837
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49931}

infra/config/cq.cfg

@@ -148,14 +148,14 @@ verifiers {
         name: "v8_win64_dbg"
         experiment_percentage: 10
       }
-      builders {
-        name: "v8_mac_gc_stress_dbg"
-        experiment_percentage: 10
-      }
-      builders {
-        name: "v8_mac64_dbg"
-        experiment_percentage: 10
-      }
+      #builders {
+      #  name: "v8_mac_gc_stress_dbg"
+      #  experiment_percentage: 10
+      #}
+      #builders {
+      #  name: "v8_mac64_dbg"
+      #  experiment_percentage: 10
+      #}
       builders {
         name: "v8_linux_gcc_rel"
         experiment_percentage: 10