Refactor the way page_set is checked in test_runner and record_wprd

tools/telemetry/telemetry/page/record_wpr.py

 #!/usr/bin/env python
 # 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.
 import json
 import logging
 import os
 import sys
 import tempfile
 import time
@@ skipping 109 matching lines @@
     parser.print_usage()
     sys.exit(1)
   target = quick_args[0]
   if target in tests:
     recorder.test = tests[target]().test()
     recorder.test.AddCommandLineArgs(parser)
     recorder.test.SetArgumentDefaults(parser)
     parser.parse_args()
     recorder.test.ProcessCommandLineArgs(parser, options)
     ps = tests[target]().CreatePageSet(options)
-  elif target.endswith('.json') or target.endswith('.py'):
+  elif discover.IsPageSetFile(target):
     parser.parse_args()
     ps = page_set.PageSet.FromFile(target)
   elif target.startswith('http'):
     parser.parse_args()
     ps = _CreatePageSetForUrl(target)
   else:
     parser.print_usage()
     sys.exit(1)
 
   page_runner.ProcessCommandLineArgs(parser, options)
@@ skipping 22 matching lines @@
                     'updated for these pages.')
     logging.warning('Skipped pages:\n%s', '\n'.join(zip(*results.skipped)[0]))
 
   if results.successes:
     # Update the metadata for the pages which were recorded.
     ps.wpr_archive_info.AddRecordedPages(results.successes)
   else:
     os.remove(temp_target_wpr_file_path)
 
   return min(255, len(results.failures))