Oilpan: move DOMImplementation to the heap.

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index ac26f1f895f217552979672df40d5784e34a3ba0..c574e621ae8b5c03b5c3d4c74dfb4f467a49a7ab 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -714,7 +714,7 @@ void Document::setDoctype(PassRefPtr<DocumentType> docType)
 DOMImplementation& Document::implementation()
 {
     if (!m_implementation)
-        m_implementation = DOMImplementation::create(*this);
+        m_implementation = DOMImplementation::create(this);
     return *m_implementation;
 }
 
@@ -5679,6 +5679,7 @@ void Document::clearWeakMembers(Visitor* visitor)
 void Document::trace(Visitor* visitor)
 {
     visitor->trace(m_docType);
+    visitor->trace(m_implementation);
     visitor->trace(m_autofocusElement);
     visitor->trace(m_focusedElement);
     visitor->trace(m_hoverNode);