[DevTools] Move extension embedder message to InspectorFrontentAPI.

Source/devtools/front_end/toolbox_bootstrap/Toolbox.js

Index: Source/devtools/front_end/toolbox_bootstrap/Toolbox.js
diff --git a/Source/devtools/front_end/toolbox_bootstrap/Toolbox.js b/Source/devtools/front_end/toolbox_bootstrap/Toolbox.js
index 10d794fdb3a393d102c9310d721db306cf4a7261..1b6419317b714254d263f2c22b864c3c4fc53302 100644
--- a/Source/devtools/front_end/toolbox_bootstrap/Toolbox.js
+++ b/Source/devtools/front_end/toolbox_bootstrap/Toolbox.js
@@ -2,11 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// FIXME: This stub is invoked from the backend and should be removed
-// once we migrate to the "pull" model for extensions retrieval.
-WebInspector = {};
-WebInspector.addExtensions = function() {};
-
 (function()
 {