Make WebGL extensions get lost when context is lost.

Source/core/html/canvas/WebGLDepthTexture.cpp

Index: Source/core/html/canvas/WebGLDepthTexture.cpp
diff --git a/Source/core/html/canvas/WebGLDepthTexture.cpp b/Source/core/html/canvas/WebGLDepthTexture.cpp
index f9b9d84a6fcea1afcc23b50f5895eccd6c88ea87..538b6fdd04e50dd02f8018c8c66431f4b491d7b5 100644
--- a/Source/core/html/canvas/WebGLDepthTexture.cpp
+++ b/Source/core/html/canvas/WebGLDepthTexture.cpp
@@ -35,13 +35,18 @@ WebGLDepthTexture::WebGLDepthTexture(WebGLRenderingContext* context)
     : WebGLExtension(context)
 {
     ScriptWrappable::init(this);
-    context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_CHROMIUM_depth_texture");
+    enable();
 }
 
 WebGLDepthTexture::~WebGLDepthTexture()
 {
 }
 
+void WebGLDepthTexture::enable()
+{
+    m_context->graphicsContext3D()->getExtensions()->ensureEnabled("GL_CHROMIUM_depth_texture");
+}
+
 WebGLExtension::ExtensionName WebGLDepthTexture::getName() const
 {
     return WebGLDepthTextureName;