Benchmark to characterize main thread usage during load

tools/perf/perf_tools/loading_benchmark.py

Index: tools/perf/perf_tools/loading_benchmark.py
diff --git a/tools/perf/perf_tools/loading_benchmark.py b/tools/perf/perf_tools/loading_benchmark.py
new file mode 100644
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+++ b/tools/perf/perf_tools/loading_benchmark.py
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+# Copyright (c) 2012 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.
+
+import collections
+
+from telemetry import multi_page_benchmark
+
+class LoadingBenchmark(multi_page_benchmark.MultiPageBenchmark):
+  @property
+  def results_are_the_same_on_every_page(self):
+    return False
+
+  def WillNavigateToPage(self, page, tab):
+    tab.timeline.Start()
+
+  def MeasurePage(self, page, tab, results):
+    # In current telemetry tests, all tests wait for DocumentComplete state.
+    #
+    # TODO(nduca): when crbug.com/168431 is fixed, modify the page sets to
+    # recognize loading as a toplevel action.
+    tab.timeline.Stop()
+
+    events = tab.timeline.timeline_model.GetAllEvents()
+
+    events_by_name = collections.defaultdict(list)
+    for e in events:
+      events_by_name[e.name].append(e)
+
+    for key, group in events_by_name.items():
+      times = [e.self_time_ms for e in group]
+      total = sum(times)
+      biggest_jank = max(times)
+      results.Add(key, 'ms', total)
+      results.Add(key + '_max', 'ms', biggest_jank)
+      results.Add(key + '_avg', 'ms', total / len(times))