Update fieldtrial_util To Handle Combined Fieldtrial Config Format

tools/perf/core/perf_benchmark.py

 # Copyright 2015 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 os
 import sys
 
 from telemetry import benchmark
 from telemetry.internal.browser import browser_finder
 
@@ skipping 41 matching lines @@
     return target_os
 
   def _GetVariationsBrowserArgs(self, finder_options):
     variations_dir = os.path.join(os.path.dirname(__file__), '..',
                                   '..', '..', 'testing', 'variations')
     possible_browser = browser_finder.FindBrowser(finder_options)
     if not possible_browser:
       return []
 
     return fieldtrial_util.GenerateArgs(
-        os.path.join(variations_dir,
-                     'fieldtrial_testing_config_%s.json' % self._FixupTargetOS(
-                         possible_browser.target_os)))
+        os.path.join(variations_dir, 'fieldtrial_testing_config.json'),
+        self._FixupTargetOS(possible_browser.target_os))
 
   @staticmethod
   def IsSvelte(possible_browser):
     """Returns whether a possible_browser is on a svelte Android build."""
     if possible_browser.target_os == 'android':
       return possible_browser.platform.IsSvelte()
     return False