Make NGFragment to own NGPhysicalFragment

third_party/WebKit/Source/core/layout/ng/ng_fragment_base.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/ng/ng_fragment_base.h"
 
 #include "core/layout/ng/ng_physical_fragment_base.h"
 
 namespace blink {
 
@@ skipping 28 matching lines @@
   return writing_mode_ == HorizontalTopBottom
              ? physical_fragment_->TopOffset()
              : physical_fragment_->LeftOffset();
 }
 
 NGMarginStrut NGFragmentBase::MarginStrut() const {
   // TODO(layout-ng): Change MarginStrut to support writing direction.
   return physical_fragment_->MarginStrut();
 }
 
-DEFINE_TRACE(NGFragmentBase) {
-  visitor->trace(physical_fragment_);
-}
-
 }  // namespace blink