Introduce EditingNG runtime enabled feature

This patch introduces "EditingNG" runtime enabled feature which implies
"LayoutNG" for adapting editing to LayoutNG incremental. Once editing
functionalities adapted to LayoutNG, this runtime enabled feature will be
removed.

Bug: 707656
Change-Id: I3e9edcf9a207e284740d082feb8ef08dbec090a9
Reviewed-on: https://chromium-review.googlesource.com/1152715Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578756}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -414,6 +414,10 @@
     {
       name: "DocumentWrite",
     },
+    {
+      // http://crbug.com/707656 content editable in LayoutNG.
+      name: "EditingNG",
+    },
     {
       name: "EmbedderCSPEnforcement",
       status: "stable",
@@ -652,7 +656,7 @@
     },
     {
       name: "LayoutNG",
-      implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFlexBox"],
+      implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFlexBox", "EditingNG"],
     },
     {
       name: "LayoutNGBlockFragmentation",