Don't walk treescopes in updateActiveStyleSheets until the resolver exists.

sky/engine/core/dom/StyleSheetCollection.h

Index: sky/engine/core/dom/StyleSheetCollection.h
diff --git a/sky/engine/core/dom/StyleSheetCollection.h b/sky/engine/core/dom/StyleSheetCollection.h
index a678582a409f79bbbcf78c9ea90f86c4fbcc81d4..7a10f95ae7d64cfeb257122b9bb492bee79d711a 100644
--- a/sky/engine/core/dom/StyleSheetCollection.h
+++ b/sky/engine/core/dom/StyleSheetCollection.h
@@ -39,7 +39,7 @@ namespace blink {
 class CSSStyleSheet;
 class ContainerNode;
 class HTMLStyleElement;
-class StyleEngine;
+class StyleResolver;
 class TreeScope;
 
 class StyleSheetCollection {
@@ -58,7 +58,7 @@ public:
     void addStyleSheetCandidateNode(HTMLStyleElement&);
     void removeStyleSheetCandidateNode(HTMLStyleElement&);
 
-    void updateActiveStyleSheets(StyleEngine*);
+    void updateActiveStyleSheets(StyleResolver&);
 
 private:
     explicit StyleSheetCollection(TreeScope&);