Enforce context refrence guards in WebGL extensions.

Source/core/html/canvas/WebGLExtension.cpp

Index: Source/core/html/canvas/WebGLExtension.cpp
diff --git a/Source/core/html/canvas/WebGLExtension.cpp b/Source/core/html/canvas/WebGLExtension.cpp
index 8070663afc4bbf6847d6acb1489dd449b0e4f202..a65b65c64ed977dd5aa08c3528aff90fcac5c2d8 100644
--- a/Source/core/html/canvas/WebGLExtension.cpp
+++ b/Source/core/html/canvas/WebGLExtension.cpp
@@ -29,6 +29,21 @@
 
 namespace blink {
 
+WebGLExtensionScopedContext::WebGLExtensionScopedContext(WebGLExtension* extension)
+    : m_context(extension->m_context)
+{
+}
+
+WebGLExtensionScopedContext::~WebGLExtensionScopedContext()
+{
+}
+
+void WebGLExtensionScopedContext::trace(Visitor* visitor)
+{
+    visitor->trace(m_context);
+}
+
+
 WebGLExtension::WebGLExtension(WebGLRenderingContextBase* context)
     : m_context(context)
 {