StyleResolver should update RuleSets lazily.

Source/core/editing/EditingStyle.cpp

Index: Source/core/editing/EditingStyle.cpp
diff --git a/Source/core/editing/EditingStyle.cpp b/Source/core/editing/EditingStyle.cpp
index 358819603f07d364642bd2b3b67c6ba9b47add9a..18ca09b768a4e4f53f2c0ab7745480fe8d6db2d5 100644
--- a/Source/core/editing/EditingStyle.cpp
+++ b/Source/core/editing/EditingStyle.cpp
@@ -1133,7 +1133,9 @@ void EditingStyle::mergeStyle(const StylePropertySet* style, CSSPropertyOverride
 static PassRefPtr<MutableStylePropertySet> styleFromMatchedRulesForElement(Element* element, unsigned rulesToInclude)
 {
     RefPtr<MutableStylePropertySet> style = MutableStylePropertySet::create();
-    RefPtr<StyleRuleList> matchedRules = element->document().styleResolver()->styleRulesForElement(element, rulesToInclude);
+    StyleResolver* styleResolver = element->document().styleResolver();
+    appendPendingStyleSheetsIfNeeded(styleResolver);
+    RefPtr<StyleRuleList> matchedRules = styleResolver->styleRulesForElement(element, rulesToInclude);
     if (matchedRules) {
         for (unsigned i = 0; i < matchedRules->m_list.size(); ++i)
             style->mergeAndOverrideOnConflict(matchedRules->m_list[i]->properties());