Expose minContentWidth/maxContentWidth and a callback for computing them.

sky/engine/core/rendering/RenderCustomLayout.cpp

Index: sky/engine/core/rendering/RenderCustomLayout.cpp
diff --git a/sky/engine/core/rendering/RenderCustomLayout.cpp b/sky/engine/core/rendering/RenderCustomLayout.cpp
index 5f55b5f51eee844ce168134df93075577ca96942..9001e37940f880dbd19ec9024aa91e9db9b2c654 100644
--- a/sky/engine/core/rendering/RenderCustomLayout.cpp
+++ b/sky/engine/core/rendering/RenderCustomLayout.cpp
@@ -19,6 +19,13 @@ RenderCustomLayout::~RenderCustomLayout()
 {
 }
 
+void RenderCustomLayout::computePreferredLogicalWidths()
+{
+    ASSERT(node()->isElementNode());
+    toElement(node())->intrinsicWidthsComputer()->handleEvent();
+    clearPreferredLogicalWidthsDirty();
+}
+
 void RenderCustomLayout::layout()
 {
     ASSERT(node()->isElementNode());