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

third_party/WebKit/Source/core/layout/ng/ng_layout_algorithm.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 NGLayoutAlgorithm_h
 #define NGLayoutAlgorithm_h
 
 #include "core/CoreExport.h"
 #include "wtf/Allocator.h"
 #include "wtf/Noncopyable.h"
 
 namespace blink {
 
 class NGConstraintSpace;
 class NGFragment;
 
 // Base class for all LayoutNG algorithms.
 class CORE_EXPORT NGLayoutAlgorithm {
   WTF_MAKE_NONCOPYABLE(NGLayoutAlgorithm);
   USING_FAST_MALLOC(NGLayoutAlgorithm);
 
  public:
   NGLayoutAlgorithm() {}
 
   // Actual layout function. Lays out the children and descendents within the
   // constraints given by the NGConstraintSpace. Returns a fragment with the
   // resulting layout information.
   // This function can not be const because for interruptible layout, we have
   // to be able to store state information.
-  virtual NGFragment* layout(const NGConstraintSpace*) = 0;
+  // 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?
+  virtual bool Layout(const NGConstraintSpace*, NGFragment**) = 0;
 };
 
 }  // namespace blink
 
 #endif  // NGLayoutAlgorithm_h