Expose minContentWidth/maxContentWidth and a callback for computing them.

sky/engine/core/rendering/RenderCustomLayout.h

 // Copyright 2015 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 SKY_ENGINE_CORE_RENDERING_RENDERCUSTOMLAYOUT_H_
 #define SKY_ENGINE_CORE_RENDERING_RENDERCUSTOMLAYOUT_H_
 
 #include "sky/engine/core/rendering/RenderBlock.h"
 
 namespace blink {
 
 class RenderCustomLayout : public RenderBlock {
 public:
     explicit RenderCustomLayout(ContainerNode* node);
-    virtual void layout() override;
+    void computePreferredLogicalWidths() final;
+    void layout() final;
     const char* renderName() const;
     bool isRenderCustomLayout() const final { return true; }
 
 protected:
     virtual ~RenderCustomLayout();
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_RENDERING_RENDERCUSTOMLAYOUT_H_