Change EditingNG to stable from experimental

This patch changes EditingNG to stable from experimental for releasing
on M89.

Bug: 707656
Change-Id: I1af3e5d9b0d0b7e5b2bc5a434936ed0eb927bd32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2557944
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830933}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -767,7 +767,7 @@
     {
       // http://crbug.com/707656 content editable in LayoutNG.
       name: "EditingNG",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "EncryptedMediaEncryptionSchemeQuery",