Change PageTest.WillStartBrowser to depend on platform instead of browser object as it would not ha…

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 metrics import power
 
 from telemetry.core.platform import tracing_category_filter
 from telemetry.core.platform import tracing_options
 from telemetry.page import page_test
 from telemetry.timeline import model
 from telemetry.value import scalar
 
 
 class ImageDecoding(page_test.PageTest):
   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, browser):
-    self._power_metric = power.PowerMetric(browser)
+  def WillStartBrowser(self, platform):
+    self._power_metric = power.PowerMetric(platform)
 
   def WillNavigateToPage(self, page, tab):
     tab.ExecuteJavaScript("""
         if (window.chrome &&
             chrome.gpuBenchmarking &&
             chrome.gpuBenchmarking.clearImageCache) {
           chrome.gpuBenchmarking.clearImageCache();
         }
     """)
     self._power_metric.Start(page, tab)
@@ skipping 60 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 CleanUpAfterPage(self, page, tab):
     tracing_controller = tab.browser.platform.tracing_controller
     if tracing_controller.is_tracing_running:
       tracing_controller.Stop()