Files app: Move common/js/externs.js to externs/*.

ui/file_manager/externs/file_operation_progress_event.js

Index: ui/file_manager/externs/file_operation_progress_event.js
diff --git a/ui/file_manager/externs/file_operation_progress_event.js b/ui/file_manager/externs/file_operation_progress_event.js
new file mode 100644
index 0000000000000000000000000000000000000000..6561d1f23f20bddf291e3a6a2c91d95c2d8baeb0
--- /dev/null
+++ b/ui/file_manager/externs/file_operation_progress_event.js
@@ -0,0 +1,15 @@
+// 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.
+
+/**
+ * @constructor
+ * @extends {Event}
+ */
+var FileOperationProgressEvent = function() {};
+
+/** @type {fileOperationUtil.EventRouter.EventType} */
+FileOperationProgressEvent.prototype.reason;
+
+/** @type {(fileOperationUtil.Error|undefined)} */
+FileOperationProgressEvent.prototype.error;