Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/EXTTextureFilterAnisotropic.cpp

Index: Source/core/html/canvas/EXTTextureFilterAnisotropic.cpp
diff --git a/Source/core/html/canvas/EXTTextureFilterAnisotropic.cpp b/Source/core/html/canvas/EXTTextureFilterAnisotropic.cpp
index 6128c259188f658e22d65216413d0e38d43f7595..50cc8d4208d655d07714ffade16365d58a65ce36 100644
--- a/Source/core/html/canvas/EXTTextureFilterAnisotropic.cpp
+++ b/Source/core/html/canvas/EXTTextureFilterAnisotropic.cpp
@@ -34,13 +34,18 @@ EXTTextureFilterAnisotropic::EXTTextureFilterAnisotropic(WebGLRenderingContext*
     : WebGLExtension(context)
 {
     ScriptWrappable::init(this);
-    context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_EXT_texture_filter_anisotropic");
+    enable();
 }
 
 EXTTextureFilterAnisotropic::~EXTTextureFilterAnisotropic()
 {
 }
 
+void EXTTextureFilterAnisotropic::enable()
+{
+    m_context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_EXT_texture_filter_anisotropic");
+}
+
 WebGLExtension::ExtensionName EXTTextureFilterAnisotropic::getName() const
 {
     return EXTTextureFilterAnisotropicName;