Expose minContentWidth/maxContentWidth and a callback for computing them.

sky/tests/layout/custom-dirty-bits.sky

 <import src="../resources/dump-as-render-tree.sky" />
 
 <block>hello</block>
 
 <script>
 import 'dart:sky';
 
 void main() {
   // This tests custom layout of text nodes inside non-paragraphs.
   // They shouldn't show up in the render tree at all, so they
   // don't need laying out.
   // This test passes if it doesn't crash and the render tree
   // has no RenderTexts.
-  document.querySelector('block').setLayoutManager(() => {});
+  document.querySelector('block').setLayoutManager(() {}, () {});
 }
 </script>