Oilpan: Remove guardRef and guardDeref from TreeScope.

Source/core/html/HTMLCanvasElement.cpp

Index: Source/core/html/HTMLCanvasElement.cpp
diff --git a/Source/core/html/HTMLCanvasElement.cpp b/Source/core/html/HTMLCanvasElement.cpp
index 54f25ea62e6ae151385ddb556821a6cb4390dd09..15886521fbe1903540cb92094a621192f424ddd6 100644
--- a/Source/core/html/HTMLCanvasElement.cpp
+++ b/Source/core/html/HTMLCanvasElement.cpp
@@ -97,7 +97,9 @@ HTMLCanvasElement::~HTMLCanvasElement()
     for (HashSet<CanvasObserver*>::iterator it = m_observers.begin(); it != end; ++it)
         (*it)->canvasDestroyed(this);
 
+#if !ENABLE(OILPAN)
     m_context.clear(); // Ensure this goes away before the ImageBuffer.
+#endif
 }
 
 void HTMLCanvasElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
@@ -529,6 +531,12 @@ void HTMLCanvasElement::notifySurfaceInvalid()
     }
 }
 
+void HTMLCanvasElement::trace(Visitor* visitor)
+{
+    visitor->trace(m_context);
+    HTMLElement::trace(visitor);
+}
+
 void HTMLCanvasElement::updateExternallyAllocatedMemory() const
 {
     int bufferCount = 0;