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 2abffdb3c203a6e83ef618e5bc0731d53d7602f7..d7ce43c2cd4a868b8317f2db9f1107f0ca83704c 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -699,11 +699,13 @@ WebGLRenderingContextBase::~WebGLRenderingContextBase()
 
     destroyContext();
 
+#if !ENABLE(OILPAN)
     if (m_multisamplingObserverRegistered) {
         Page* page = canvas()->document().page();
         if (page)
             page->removeMultisamplingChangedObserver(this);
     }
+#endif
 
     willDestroyContext(this);
 }