Expose minContentWidth/maxContentWidth and a callback for computing them.

sky/engine/core/rendering/RenderCustomLayout.h

Index: sky/engine/core/rendering/RenderCustomLayout.h
diff --git a/sky/engine/core/rendering/RenderCustomLayout.h b/sky/engine/core/rendering/RenderCustomLayout.h
index 0b1c5da42134db2cfc065aa45882f31924d7a43d..826a787fb42fe4adb641da8d670ba47a7bc755f6 100644
--- a/sky/engine/core/rendering/RenderCustomLayout.h
+++ b/sky/engine/core/rendering/RenderCustomLayout.h
@@ -12,7 +12,8 @@ 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; }