predictors: Fix prefetch benchmark.

tools/resource_prefetch_predictor/prefetch_benchmark.py

 #!/usr/bin/python
 #
 # Copyright 2016 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.
 
 """Loads a web page with speculative prefetch, and collects loading metrics."""
 
 import argparse
 import logging
@@ skipping 82 matching lines @@
 
 
 def _RunOnce(device, database_filename, url, prefetch_delay_ms,
              output_filename, wpr_archive, network_condition):
   _Setup(device, database_filename)
 
   disable_prefetch = prefetch_delay_ms == -1
   # Startup tracing to ease debugging.
   chrome_args = (customtabs_benchmark.CHROME_ARGS
                  + ['--trace-startup', '--trace-startup-duration=20'])
-  if not disable_prefetch:
-    chrome_args.append(_EXTERNAL_PREFETCH_FLAG)
 
   chrome_controller = controller.RemoteChromeController(device)
   device.ForceStop(OPTIONS.ChromePackage().package)
   chrome_controller.AddChromeArguments(chrome_args)
 
   with device_setup.RemoteWprHost(
       device, wpr_archive, record=False,
       network_condition_name=network_condition) as wpr:
     logging.info('WPR arguments: ' +  ' '.join(wpr.chrome_args))
     chrome_args += wpr.chrome_args
@@ skipping 34 matching lines @@
   while True:
     delay = delays[random.randint(0, len(delays) - 1)]
     _RunOnce(device, args.database, args.url, delay, args.output_filename,
              args.wpr_archive, args.network_condition)
     if args.once:
       return
 
 
 if __name__ == '__main__':
   main()