Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/OESStandardDerivatives.cpp

Index: Source/core/html/canvas/OESStandardDerivatives.cpp
diff --git a/Source/core/html/canvas/OESStandardDerivatives.cpp b/Source/core/html/canvas/OESStandardDerivatives.cpp
index 988c878d34ed6ea0a2141cf6bb557bd90f62e8e0..95f48d3769f02640c15f7f8e9ec391e961ae911c 100644
--- a/Source/core/html/canvas/OESStandardDerivatives.cpp
+++ b/Source/core/html/canvas/OESStandardDerivatives.cpp
@@ -34,13 +34,17 @@ OESStandardDerivatives::OESStandardDerivatives(WebGLRenderingContext* context)
     : WebGLExtension(context)
 {
     ScriptWrappable::init(this);

[Ken Russell (switch to Gerrit), 2013/06/05 00:46:38]

Missing call to enable()?

-    context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_OES_standard_derivatives");
 }
 
 OESStandardDerivatives::~OESStandardDerivatives()
 {
 }
 
+void OESStandardDerivatives::enable()
+{
+    m_context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_OES_standard_derivatives");
+}
+
 WebGLExtension::ExtensionName OESStandardDerivatives::getName() const
 {
     return OESStandardDerivativesName;