Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/OESElementIndexUint.cpp

Index: Source/core/html/canvas/OESElementIndexUint.cpp
diff --git a/Source/core/html/canvas/OESElementIndexUint.cpp b/Source/core/html/canvas/OESElementIndexUint.cpp
index be10812b5008971bd6f6038e6d82b03ba51f8d2d..4f6880946ab3fadf51d285df0c7fba2e891966d9 100644
--- a/Source/core/html/canvas/OESElementIndexUint.cpp
+++ b/Source/core/html/canvas/OESElementIndexUint.cpp
@@ -34,13 +34,18 @@ OESElementIndexUint::OESElementIndexUint(WebGLRenderingContext* context)
     : WebGLExtension(context)
 {
     ScriptWrappable::init(this);
-    context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_OES_element_index_uint");
+    enable();
 }
 
 OESElementIndexUint::~OESElementIndexUint()
 {
 }
 
+void OESElementIndexUint::enable()
+{
+    m_context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_OES_element_index_uint");
+}
+
 WebGLExtension::ExtensionName OESElementIndexUint::getName() const
 {
     return OESElementIndexUintName;