[tools/perf] Fix benchmark smoke tests to not set story_set.stories directly

tools/perf/benchmarks/system_health_smoke_test.py

 # 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.
 
 """Run all system health stories used by system health benchmarks.
 
 Only memory benchmarks are used when running these stories to make the total
 cycle time manageable. Other system health benchmarks should be using the same
 stories as memory ones, only with fewer actions (no memory dumping).
 """
@@ skipping 52 matching lines @@
   # disabling it for one failing or flaky benchmark would disable a much
   # wider swath of coverage  than is usally intended. Instead, if a test is
   # failing, disable it by putting it into the _DISABLED_TESTS list above.
   @benchmark_module.Disabled('chromeos')  # crbug.com/351114
   def RunTest(self):
 
     class SinglePageBenchmark(benchmark_class):  # pylint: disable=no-init
       def CreateStorySet(self, options):
         # pylint: disable=super-on-old-class
         story_set = super(SinglePageBenchmark, self).CreateStorySet(options)
-        assert story_to_smoke_test in story_set.stories
-        story_set.stories = [story_to_smoke_test]
+        stories_to_remove = [s for s in story_set.stories if s !=
+                             story_to_smoke_test]
+        for s in stories_to_remove:
+          story_set.RemoveStory(s)
+        assert story_set.stories
         return story_set
 
     options = GenerateBenchmarkOptions(benchmark_class)
     possible_browser = browser_finder.FindBrowser(options)
     if possible_browser is None:
       self.skipTest('Cannot find the browser to run the test.')
     if (SinglePageBenchmark.ShouldDisable(possible_browser) or
         not decorators.IsEnabled(benchmark_class, possible_browser)[0]):
       self.skipTest('Benchmark %s is disabled' % SinglePageBenchmark.Name())
 
@@ skipping 55 matching lines @@
     # parsed during test time which happens after load_tests are called.
     # Since none of our system health benchmarks creates stories based on
     # command line options, it should be ok to pass options=None to
     # CreateStorySet.
     for story_to_smoke_test in (
         benchmark_class().CreateStorySet(options=None).stories):
       suite.addTest(
           _GenerateSmokeTestCase(benchmark_class, story_to_smoke_test))
 
   return suite