TextControl NG: Make LayoutNGTextArea flag "experimental"

Bug: 1040826
Change-Id: If834f99a2d97c295cbe0e574538b891533992443
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494182
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820116}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1055,7 +1055,7 @@
     {
       name: "LayoutNGTextArea",
       depends_on: ["EditingNG"],
-      status: "test",
+      status: "experimental",
     },
     {
       name: "LayoutNGTextField",