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

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

Index: third_party/WebKit/Source/core/layout/ng/ng_box.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_box.h b/third_party/WebKit/Source/core/layout/ng/ng_box.h
index 7eab453de5f81f1e0c586f2cbf06906278d22bd4..a30c63352445ed05d55d8c2eafbce3552260d6d8 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_box.h
+++ b/third_party/WebKit/Source/core/layout/ng/ng_box.h
@@ -29,7 +29,12 @@ class CORE_EXPORT NGBox final {
   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;