Move stash_backend into extensions/browser/mojo.

extensions/common/mojo/stash.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module extensions {
 
 // A stashed client object. This contains the serialized bytes and handles,
 // stored in |data| and |handles|, respectively, of a mojo struct of the
 // serialized representation of the client object.
 struct StashedObject {
@@ skipping 12 matching lines @@
   // Adds |stashed_objects| to the stash. StashedObjects are stored in memory in
   // the browser process for as long as the extension that owns them remains
   // enabled and the browser process runs.
   AddToStash(array<StashedObject> stashed_objects);
 
   // Returns all stashed objects.
   RetrieveStash() => (array<StashedObject> stash);
 };
 
 }