Expose minContentWidth/maxContentWidth and a callback for computing them.

sky/engine/core/rendering/RenderBox.h

Index: sky/engine/core/rendering/RenderBox.h
diff --git a/sky/engine/core/rendering/RenderBox.h b/sky/engine/core/rendering/RenderBox.h
index 0361c04a114638d283430049d8792161d3712bdb..bbf8283a8eabb5436b7a526fb5b3cc782ffaaf1f 100644
--- a/sky/engine/core/rendering/RenderBox.h
+++ b/sky/engine/core/rendering/RenderBox.h
@@ -301,6 +301,9 @@ public:
     virtual LayoutUnit minPreferredLogicalWidth() const override;
     virtual LayoutUnit maxPreferredLogicalWidth() const override;
 
+    void setMinPreferredLogicalWidth(LayoutUnit);
+    void setMaxPreferredLogicalWidth(LayoutUnit);
+
     // FIXME: We should rename these back to overrideLogicalHeight/Width and have them store
     // the border-box height/width like the regular height/width accessors on RenderBox.
     // Right now, these are different than contentHeight/contentWidth because they still