[layoutng] async layout part 1: Change the API to support async

third_party/WebKit/Source/core/layout/ng/ng_box.h

 // 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.
 
 #ifndef NGBox_h
 #define NGBox_h
 
 #include "core/layout/LayoutBox.h"
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
@@ skipping 11 matching lines @@
  public:
   explicit NGBox(LayoutObject* layoutObject)
       : m_layoutBox(toLayoutBox(layoutObject)) {}
 
   NGBox() : m_layoutBox(nullptr) {}
 
   // Returns an iterator that will iterate over this box's children, if any.
   NGBoxIterator childIterator();
   operator bool() const { return m_layoutBox; }
 
-  NGFragment* layout(const NGConstraintSpace*);
+  // Returns true when done; when this function returns false, it has to be
+  // called again. The out parameter will only be set when this function
+  // returns true. The same constraint space has to be passed each time.
+  // TODO(layout-ng): Should we have a StartLayout function to avoid passing
+  // the same space for each Layout iteration?
+  bool Layout(const NGConstraintSpace*, NGFragment**);
   const ComputedStyle* style() const;
 
   NGBox nextSibling() const;
 
   NGBox firstChild() const;
 
   // This is necessary for interop between old and new trees -- after our parent
   // positions us, it calls this function so we can store the position on the
   // underlying LayoutBox.
   void positionUpdated(const NGFragment&);
 
  private:
   bool canUseNewLayout();
 
   LayoutBox* m_layoutBox;
 };
 
 }  // namespace blink
 
 #endif  // NGBox_h