[telemetry] Clean up image decoding benchmark

tools/perf/perf_tools/image_decoding_benchmark.py

 # 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.
 
 from telemetry import multi_page_benchmark
-from telemetry import util
 
 
 class ImageDecoding(multi_page_benchmark.MultiPageBenchmark):
-  def MeasurePage(self, _, tab, results):
+  def WillNavigateToPage(self, page, tab):
+    tab.timeline.Start()
+
+  def MeasurePage(self, page, tab, results):
+    tab.timeline.Stop()
     def _IsDone():
       return tab.runtime.Evaluate('isDone')
 
-    with tab.timeline.Recorder(tab.timeline):
-      tab.runtime.Execute('runBenchmark()')
-      util.WaitFor(_IsDone, 60)
-    iterations = tab.runtime.Evaluate('minIterations')
-    decode_image = tab.timeline.timeline_events.GetAllOfType('DecodeImage')
-    elapsed_times = [d.elapsed_time for d in decode_image[-iterations:]]
+    decode_image_events = \
+        tab.timeline.timeline_events.GetAllOfType('DecodeImage')
+
+    # If it is a real image benchmark, then store only the last-minIterations
+    # decode tasks.
+    if (hasattr(page, 'is_image_decoding_benchmark') and
+        page.is_image_decoding_benchmark):
+      assert _IsDone()
+      max_event_count = tab.runtime.Evaluate('minIterations')
+      decode_image_events = decode_image_events[-max_event_count:]
+
+    elapsed_times = [d.elapsed_time for d in decode_image_events]
     if not elapsed_times:
       results.Add('ImageDecoding_avg', 'ms', 'unsupported')
       return
     image_decoding_avg = sum(elapsed_times) / len(elapsed_times)
     results.Add('ImageDecoding_avg', 'ms', image_decoding_avg)