[FlexNG] Move "-webkit-box" (non line-clamp) over to FlexNG.

As above. Previously this was handled by LayoutFlexibleBox.

Bug: 845235
Change-Id: Id4899b487c92b723341c8251601f317abb6618c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451732Reviewed-by: David Grogan <dgrogan@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814361}

third_party/blink/renderer/core/layout/layout_object.cc

@@ -272,7 +272,8 @@ LayoutObject* LayoutObject::CreateObject(Element* element,
         return LayoutObjectFactory::CreateBlockForLineClamp(*element, style,
                                                             legacy);
       }
-
+      if (RuntimeEnabledFeatures::LayoutNGWebkitBoxEnabled())
+        return LayoutObjectFactory::CreateFlexibleBox(*element, style, legacy);
       UseCounter::Count(element->GetDocument(),
                         WebFeature::kLegacyLayoutByFlexBox);
       return new LayoutFlexibleBox(element);

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1037,6 +1037,10 @@
       name: "LayoutNGTextField",
       depends_on: ["EditingNG"],
     },
+    {
+      name: "LayoutNGWebkitBox",
+      status: "stable",
+    },
     {
       // Adds the "sources" attribute to the LayoutShift interface for the
       // Layout Instability API (bit.ly/cls-sources).