[Telemetry] Make page cyclers more telemetric.

tools/perf/perf_tools/page_cycler.js

+// Copyright (c) 2013 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.
+
+(function() {
+// The Mozilla DHTML performance tests need to explicitly call a function to
+// trigger the next page visit, rather than directly using the onload handler.
+// To meet needs of the DHTML performance tests without forking this head.js
+// file, use a variable |__install_onload_handler| to indicate whether the
+// |__onload| handler should be added to onload event listener.
+// Install |__onload| by default if there is no pre-configuration.
+if (typeof(__install_onload_handler) == 'undefined')
+  var __install_onload_handler = true;
+
+// This is the timeout used in setTimeout inside the DHTML tests.  Chrome has
+// a much more accurate timer resolution than other browsers do.  This results
+// in Chrome running these tests much faster than other browsers.  In order to
+// compare Chrome with other browsers on DHTML performance alone, set this
+// value to ~15.
+var __test_timeout = 0;
+
+function __set_cookie(name, value) {
+  document.cookie = name + "=" + value + "; path=/";
+}
+
+function __onbeforeunload() {
+  // Call GC twice to cleanup JS heap before starting a new test.
+  if (window.gc) {
+    window.gc();
+    window.gc();
+  }
+
+  __set_cookie("__pc_load_time", "");
+}
+
+// The function |__onload| is used by the DHTML tests.
+window.__onload = function() {
+  // window.storeCreated is used by the Indexed DB tests.
+  if ((!__install_onload_handler || window.storeCreated) &&
+      !performance.timing.loadEventEnd)
+    return;
+
+  var unused = document.body.offsetHeight;  // force layout
+
+  __set_cookie("__pc_load_time", window.performance.now());
+};
+
+// The function |testComplete| is used by the Indexed DB tests.
+window.testComplete = __onload;
+
+// The function |__eval_later| now is only used by the DHTML tests.
+window.__eval_later = function(expression) {
+  setTimeout(expression, __test_timeout);
+};
+
+if (window.parent == window) {  // Ignore subframes.
+  addEventListener("load", __onload);
+  addEventListener("beforeunload", __onbeforeunload);
+}
+})();