Custom state: Make the feature experimental state

Bug: 1012098
Change-Id: I266825bc3482fbc82c00fa9ef96d7b269c03a4fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1852345Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704560}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -514,7 +514,7 @@
     },
     {
       name: "CustomStatePseudoClass",
-      status: "test",
+      status: "experimental",
     },
     {
       name: "Database",