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 a39ff73483b76d1be2e00c225771ce5d5fa4611f..1e58be7ba4979ac6ef4b317d8aa35105ee36bff6 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -697,11 +697,13 @@ WebGLRenderingContextBase::~WebGLRenderingContextBase()
 
     destroyContext();
 
+#if !ENABLE(OILPAN)
     if (m_multisamplingObserverRegistered) {
         Page* page = canvas()->document().page();
         if (page)
             page->removeMultisamplingChangedObserver(this);
     }
+#endif
 
     willDestroyContext(this);
 }