Oilpan: Make ActiveParserSession::m_document traceable.

Source/core/html/parser/HTMLParserScheduler.h

Index: Source/core/html/parser/HTMLParserScheduler.h
diff --git a/Source/core/html/parser/HTMLParserScheduler.h b/Source/core/html/parser/HTMLParserScheduler.h
index da4336d414f52796b1da9a8f147cb6a53f2ee234..4e3364edfbffe48bdd9cfb543ce2247800fd0a86 100644
--- a/Source/core/html/parser/HTMLParserScheduler.h
+++ b/Source/core/html/parser/HTMLParserScheduler.h
@@ -38,15 +38,17 @@ class Document;
 class HTMLDocumentParser;
 
 class ActiveParserSession {
+    STACK_ALLOCATED();
 public:
     explicit ActiveParserSession(Document*);
     ~ActiveParserSession();
 
 private:
-    RefPtr<Document> m_document;
+    RefPtrWillBeMember<Document> m_document;
 };
 
 class PumpSession : public NestingLevelIncrementer, public ActiveParserSession {
+    STACK_ALLOCATED();
 public:
     PumpSession(unsigned& nestingLevel, Document*);
     ~PumpSession();