Enable LayoutNGFragmentItem to "stable"

Bug: 982194
Change-Id: Ieb7d92ccad9e02dee3ef1beeafe2332ec89867ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2269454Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803221}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1009,7 +1009,7 @@
     {
       name: "LayoutNGFragmentItem",
       implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFragmentTraversal", "EditingNG"],
-      status: "test",
+      status: "stable",
     },
     {
       // Traverse the fragment tree when painting and hit-testing, instead of