Move stash_backend into extensions/browser/mojo.

extensions/common/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 {
-  // A client identifier for this stashed object. This is to allow the client to
-  // reconstitute the correct serialization struct from this StashedObject.
-  string id;
-
-  // The serialized data of the serialization struct.
-  array<uint8> data;
-
-  // The handles contained within the serialization struct.
-  array<handle> stashed_handles;
-};
-
-interface StashService {
-  // 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);
-};
-
-}