Enforce context refrence guards in WebGL extensions.

Source/core/html/canvas/WebGLLoseContext.cpp

Index: Source/core/html/canvas/WebGLLoseContext.cpp
diff --git a/Source/core/html/canvas/WebGLLoseContext.cpp b/Source/core/html/canvas/WebGLLoseContext.cpp
index 7fe322272f7fffb20f6689c45e9dab42ba228dec..51e967749b981eb1e7c6ea2a2ac41eb7c9d4bfe9 100644
--- a/Source/core/html/canvas/WebGLLoseContext.cpp
+++ b/Source/core/html/canvas/WebGLLoseContext.cpp
@@ -58,14 +58,16 @@ PassRefPtrWillBeRawPtr<WebGLLoseContext> WebGLLoseContext::create(WebGLRendering
 
 void WebGLLoseContext::loseContext()
 {
-    if (!isLost())
-        m_context->forceLostContext(WebGLRenderingContextBase::WebGLLoseContextLostContext, WebGLRenderingContextBase::Manual);
+    WebGLExtensionScopedContext scoped(this);
+    if (!scoped.isLost())
+        scoped.context()->forceLostContext(WebGLRenderingContextBase::WebGLLoseContextLostContext, WebGLRenderingContextBase::Manual);
 }
 
 void WebGLLoseContext::restoreContext()
 {
-    if (!isLost())
-        m_context->forceRestoreContext();
+    WebGLExtensionScopedContext scoped(this);
+    if (!scoped.isLost())
+        scoped.context()->forceRestoreContext();
 }
 
 bool WebGLLoseContext::supported(WebGLRenderingContextBase*)