[tools/perf] Reference legacy_page_test module instead of page_test

tools/perf/measurements/image_decoding.py

 # Copyright 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.page import page_test
+from telemetry.page import legacy_page_test
 from telemetry.timeline import model
 from telemetry.timeline import tracing_config
 from telemetry.value import scalar
 
 from metrics import power
 
 
-class ImageDecoding(page_test.PageTest):
+class ImageDecoding(legacy_page_test.LegacyPageTest):
 
   def __init__(self):
     super(ImageDecoding, self).__init__()
     self._power_metric = None
 
   def CustomizeBrowserOptions(self, options):
     options.AppendExtraBrowserArgs('--enable-gpu-benchmarking')
     power.PowerMetric.CustomizeBrowserOptions(options)
 
   def WillStartBrowser(self, platform):
@@ skipping 66 matching lines @@
                     'formats: gif, png, jpg, and webp. The image files are '
                     'located at chrome/test/data/image_decoding.'))
     results.AddValue(scalar.ScalarValue(
         results.current_page, 'ImageLoading_avg', 'ms',
         tab.EvaluateJavaScript('averageLoadingTimeMs()')))
 
   def DidRunPage(self, platform):
     self._power_metric.Close()
     if platform.tracing_controller.is_tracing_running:
       platform.tracing_controller.StopTracing()