Expose minContentWidth/maxContentWidth and a callback for computing them.

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

Index: sky/tests/layout/custom-dirty-bits.sky
diff --git a/sky/tests/layout/custom-dirty-bits.sky b/sky/tests/layout/custom-dirty-bits.sky
index ec5b750956d42d862cec128feeab4f6dda97e097..d8ecf25085cc268e1186cf143fb503867b7b47a0 100644
--- a/sky/tests/layout/custom-dirty-bits.sky
+++ b/sky/tests/layout/custom-dirty-bits.sky
@@ -11,6 +11,6 @@ void main() {
   // 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>