Make a Runtime flag, ManualSlotting, set experimental

Currently the Runtime flag is setted as test, change it experimental instead
for the test in local.

Change-Id: I633fc63720022214e4857395763843db9c1d578a
Reviewed-on: https://chromium-review.googlesource.com/1212203Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Kyoko Muto <kymuto@google.com>
Cr-Commit-Position: refs/heads/master@{#589447}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -674,7 +674,7 @@
     },
     {
       name:"ManualSlotting",
-      status:"test",
+      status:"experimental",
     },
     {
       name: "MediaCapabilities",