[layoutng] Reduce virtual calls to AddChild

R=eae@chromium.org, ikilpatrick@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I42ace891fcc66faa21a21d33c18618911c31327f
Reviewed-on: https://chromium-review.googlesource.com/c/1283841Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600195}

third_party/blink/renderer/core/layout/ng/ng_container_fragment_builder.h

@@ -87,8 +87,8 @@ class CORE_EXPORT NGContainerFragmentBuilder : public NGBaseFragmentBuilder {
     return *this;
   }
 
-  virtual NGContainerFragmentBuilder& AddChild(const NGLayoutResult&,
-                                               const NGLogicalOffset&);
+  NGContainerFragmentBuilder& AddChild(const NGLayoutResult&,
+                                       const NGLogicalOffset&);
 
   // This version of AddChild will not propagate floats/out_of_flow.
   // Use the AddChild(NGLayoutResult) variant if NGLayoutResult is available.