Files App: Add metrics for background performance.

ui/file_manager/file_manager/background/js/metrics_start.js

Index: ui/file_manager/file_manager/background/js/metrics_start.js
diff --git a/ui/file_manager/file_manager/background/js/metrics_start.js b/ui/file_manager/file_manager/background/js/metrics_start.js
new file mode 100644
index 0000000000000000000000000000000000000000..e3f75ca19f16c40202cd5aa3bd9b5a38ff89f8a5
--- /dev/null
+++ b/ui/file_manager/file_manager/background/js/metrics_start.js
@@ -0,0 +1,11 @@
+// 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.
+
+/**
+ * @fileoverview Metrics calls to start measurement of script loading.  Include
+ * this as the first script in background_scripts.js (i.e. after the common
+ * scripts that define the metrics namespace).
+ */
+
+metrics.startInterval('Load.BackgroundScript');