Oilpan: Remove guardRef and guardDeref from TreeScope.

Source/core/css/StyleSheetList.cpp

Index: Source/core/css/StyleSheetList.cpp
diff --git a/Source/core/css/StyleSheetList.cpp b/Source/core/css/StyleSheetList.cpp
index 4581cb1d07048c0ca0f1cd2cf451dff464ef441f..3a648795642119b03818b1a5566d976f165de0d2 100644
--- a/Source/core/css/StyleSheetList.cpp
+++ b/Source/core/css/StyleSheetList.cpp
@@ -55,7 +55,7 @@ void StyleSheetList::detachFromDocument()
     // the StyleSheetList and the document to die together and get rid
     // of the detachedStyleSheets.
     m_detachedStyleSheets = document()->styleEngine()->styleSheetsForStyleSheetList(*m_treeScope);
-    m_treeScope = 0;
+    m_treeScope = nullptr;
 }
 
 unsigned StyleSheetList::length()
@@ -94,6 +94,7 @@ CSSStyleSheet* StyleSheetList::anonymousNamedGetter(const AtomicString& name)
 
 void StyleSheetList::trace(Visitor* visitor)
 {
+    visitor->trace(m_treeScope);
     visitor->trace(m_detachedStyleSheets);
 }