Dirty tree scopes are always a subset of active tree scopes.

third_party/WebKit/Source/core/dom/StyleEngine.cpp

Index: third_party/WebKit/Source/core/dom/StyleEngine.cpp
diff --git a/third_party/WebKit/Source/core/dom/StyleEngine.cpp b/third_party/WebKit/Source/core/dom/StyleEngine.cpp
index 62bc9c87459e6e449bcdef656b3aefe92adfbe1b..f01a0dcd72abd73f639f0c179302567a1149c398 100644
--- a/third_party/WebKit/Source/core/dom/StyleEngine.cpp
+++ b/third_party/WebKit/Source/core/dom/StyleEngine.cpp
@@ -93,6 +93,7 @@ void StyleEngine::detachFromDocument()
     m_fontSelector.clear();
     m_resolver.clear();
     m_styleSheetCollectionMap.clear();
+    m_dirtyTreeScopes.clear();
     m_activeTreeScopes.clear();
 }
 #endif
@@ -271,7 +272,6 @@ void StyleEngine::clearMediaQueryRuleSetStyleSheets()
 {
     documentStyleSheetCollection()->clearMediaQueryRuleSetStyleSheets();
     clearMediaQueryRuleSetOnTreeScopeStyleSheets(m_activeTreeScopes);
-    clearMediaQueryRuleSetOnTreeScopeStyleSheets(m_dirtyTreeScopes);
 }
 
 void StyleEngine::updateStyleSheetsInImport(DocumentStyleSheetCollector& parentCollector)