Clear CSSGlobalRuleSet on StyleEngine::didDetach().

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 045bf79748bfd48c6d0a71cdcd18fe9ef7bc4256..b5af1480b0c2bdb212949ff1d9f2de946d283877 100644
--- a/third_party/WebKit/Source/core/dom/StyleEngine.cpp
+++ b/third_party/WebKit/Source/core/dom/StyleEngine.cpp
@@ -457,6 +457,7 @@ void StyleEngine::clearResolver() {
 }
 
 void StyleEngine::didDetach() {
+  m_globalRuleSet.dispose();
   clearResolver();
   m_viewportResolver = nullptr;
   m_mediaQueryEvaluator = nullptr;