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

tools/perf/benchmarks/benchmark_smoke_unittest.py

 # Copyright 2014 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 the first page of one benchmark for every module.
 
 Only benchmarks that have a composable measurement are included.
 Ideally this test would be comprehensive, however, running one page
 of every benchmark would run impractically long.
 """
@@ skipping 35 matching lines @@
   def BenchmarkSmokeTest(self):
     # Only measure a single page so that this test cycles reasonably quickly.
     benchmark.options['pageset_repeat'] = 1
     benchmark.options['page_repeat'] = 1
 
     class SinglePageBenchmark(benchmark):  # pylint: disable=no-init
 
       def CreateStorySet(self, options):
         # pylint: disable=super-on-old-class
         story_set = super(SinglePageBenchmark, self).CreateStorySet(options)
-        for story in story_set.stories:
-          story_set.stories = [story]
-          break
+        # Only smoke test the first story since smoke testing everything takes
+        # too long.
+        for s in story_set.stories[1:]:
+          story_set.RemoveStory(s)
         return story_set
 
     # Set the benchmark's default arguments.
     options = options_for_unittests.GetCopy()
     options.output_formats = ['none']
     parser = options.CreateParser()
 
     benchmark.AddCommandLineArgs(parser)
     benchmark_module.AddCommandLineArgs(parser)
     benchmark.SetArgumentDefaults(parser)
@@ skipping 82 matching lines @@
     MergeDecorators(method, disabled_method_attr, benchmark,
                     disabled_benchmark_attr)
     MergeDecorators(method, enabled_method_attr, benchmark,
                     enabled_benchmark_attr)
 
     setattr(BenchmarkSmokeTest, benchmark.Name(), method)
 
     suite.addTest(BenchmarkSmokeTest(benchmark.Name()))
 
   return suite