Change dependency graph of runtime time enabled feature LayoutNGFragmentItem

This patch changes dependency graph of |LayoutNGFragmentItem| to make
|ScopedLayoutNGForTest()| to work with both LayoutNG and legacy layout for
running blink units for legacy layout tree.

When we use "implied_by", |LayoutNGEnabled()| is implemented as:
  static bool LayoutNGEnabled() {
    if (LayoutNGFragmentItemEnabled())
      return true;
    ... features in "implied_by".
    return is_layout_ng_enabled_;
  }

Change-Id: Id65a621bc6b041755b7897f9e5698aff26369370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275952
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784291}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -931,7 +931,7 @@
       // provides a convenient way for testing legacy layout code path in blink
       // unit tests.
       name: "LayoutNG",
-      implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFieldset", "LayoutNGFragmentItem", "LayoutNGGrid", "EditingNG", "BidiCaretAffinity", "LayoutNGTable", "LayoutNGFragmentTraversal"],
+      implied_by: ["LayoutNGFieldset", "LayoutNGGrid", "EditingNG", "BidiCaretAffinity", "LayoutNGTable"],
       status: "stable",
     },
     {