Add OBB mounter Mojo interface

components/arc/common/obb_mounter.mojom

+// Copyright 2016 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 arc.mojom;
+
+interface ObbMounterHost {
+  // Mounts the specified OBB file to the target path, the owner GID will be set
+  // to the specified value.
+  MountObb(string obb_file,
+           string target_path,
+           int32 owner_gid) => (bool success);
+
+  // Unmounts the OBB file mounted to the specifed path.
+  UnmountObb(string target_path) => (bool success);
+};
+
+interface ObbMounterInstance {
+  Init(ObbMounterHost host_ptr);
+};