Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/WebGLCompressedTextureS3TC.cpp

Index: Source/core/html/canvas/WebGLCompressedTextureS3TC.cpp
diff --git a/Source/core/html/canvas/WebGLCompressedTextureS3TC.cpp b/Source/core/html/canvas/WebGLCompressedTextureS3TC.cpp
index ae7abe34251b9196da1770edbffa0ede08ca9859..97fd085ff1f792fa58423d71fc2a20062b6aa4be 100644
--- a/Source/core/html/canvas/WebGLCompressedTextureS3TC.cpp
+++ b/Source/core/html/canvas/WebGLCompressedTextureS3TC.cpp
@@ -39,16 +39,21 @@ WebGLCompressedTextureS3TC::WebGLCompressedTextureS3TC(WebGLRenderingContext* co
     : WebGLExtension(context)
 {
     ScriptWrappable::init(this);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGB_S3TC_DXT1_EXT);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_S3TC_DXT1_EXT);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_S3TC_DXT3_EXT);
-    context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_S3TC_DXT5_EXT);
+    enable();
 }
 
 WebGLCompressedTextureS3TC::~WebGLCompressedTextureS3TC()
 {
 }
 
+void WebGLCompressedTextureS3TC::enable()
+{
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGB_S3TC_DXT1_EXT);
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_S3TC_DXT1_EXT);
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_S3TC_DXT3_EXT);
+    m_context->addCompressedTextureFormat(Extensions3D::COMPRESSED_RGBA_S3TC_DXT5_EXT);
+}
+
 WebGLExtension::ExtensionName WebGLCompressedTextureS3TC::getName() const
 {
     return WebGLCompressedTextureS3TCName;