power_LoadTest: Add parameter for custom tasks

client/site_tests/power_LoadTest/extension/params.js

 // Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-var test_time_ms = 60 * 60 * 1000;
-var test_startup_delay = 5 * 1000;
+// Convert seconds to milliseconds
+function seconds(s) {
+    return s * 1000;
+}
+
+// Convert minutes to milliseconds
+function minutes(m) {
+    return seconds(m * 60);
+}
+
+var test_time_ms = minutes(60);
+var test_startup_delay = seconds(5);
 var should_scroll = true;
 var should_scroll_up = true;
 var scroll_loop = false;
 var scroll_interval_ms = 1000;
 var scroll_by_pixels = 600;