Add benchmark that tests garbage collector and idle time handler.

tools/perf/page_sets/idle_multi_tab_cases.js

+// Copyright 2015 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.
+
+__telemetry_spawned_tabs = [];
+
+function __telemetry_spawn_tab() {
+  var tab = window.open(window.location.href);
+  if (typeof tab === "undefined") {
+    throw "Cannot open tab. Forgot --disable-popup-blocking flag?";
+  }
+  __telemetry_spawned_tabs.push(tab);
+  return __telemetry_spawned_tabs.length - 1;
+}
+
+function __telemetry_close_tab(tab) {
+  __telemetry_spawned_tabs[tab].close();
+  __telemetry_spawned_tabs[tab] = null;
+}