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

tools/perf/benchmarks/system_health_smoke_test.py

Index: tools/perf/benchmarks/system_health_smoke_test.py
diff --git a/tools/perf/benchmarks/system_health_smoke_test.py b/tools/perf/benchmarks/system_health_smoke_test.py
index 74026c97acf82726e66fd24637cf7449beeb9192..2f7d31f00ad5741b5a9a48f32f8db2524adebe07 100644
--- a/tools/perf/benchmarks/system_health_smoke_test.py
+++ b/tools/perf/benchmarks/system_health_smoke_test.py
@@ -70,8 +70,11 @@ def _GenerateSmokeTestCase(benchmark_class, story_to_smoke_test):
       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)