Oilpan: Remove guardRef and guardDeref from TreeScope.

Source/core/html/canvas/WebGLRenderingContextBase.cpp

Index: Source/core/html/canvas/WebGLRenderingContextBase.cpp
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.cpp b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
index 1d515ad3835ca1a6a3c58e56511bcc77ba0cd531..ed3f02c5a4e7a48a334d62d5a6c140ac50649c4e 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -701,11 +701,13 @@ WebGLRenderingContextBase::~WebGLRenderingContextBase()
 
     destroyContext();
 
+#if !ENABLE(OILPAN)
     if (m_multisamplingObserverRegistered) {
         Page* page = canvas()->document().page();
         if (page)
             page->removeMultisamplingChangedObserver(this);
     }
+#endif
 
     willDestroyContext(this);
 }