Create classes_util API, change discover to return a list instead of a dict.

tools/perf/benchmarks/benchmark_smoke_unittest.py

Index: tools/perf/benchmarks/benchmark_smoke_unittest.py
diff --git a/tools/perf/benchmarks/benchmark_smoke_unittest.py b/tools/perf/benchmarks/benchmark_smoke_unittest.py
index 4acd33222bc2aa0b2c9573eb712b4ce3d976d4e8..369b2879864cda4de87c0b1ec5ebd85ce561adb7 100644
--- a/tools/perf/benchmarks/benchmark_smoke_unittest.py
+++ b/tools/perf/benchmarks/benchmark_smoke_unittest.py
@@ -100,11 +100,11 @@ def load_tests(loader, standard_tests, pattern):
   benchmarks_dir = os.path.dirname(__file__)
   top_level_dir = os.path.dirname(benchmarks_dir)
 
-  # Using the default of |index_by_class_name=False| means that if a module
-  # has multiple benchmarks, only the last one is returned.
+  # Using the default of |one_class_per_module=True| means that if a module
+  # has multiple benchmarks, only the first one is returned.
   all_benchmarks = discover.DiscoverClasses(
       benchmarks_dir, top_level_dir, benchmark_module.Benchmark,
-      index_by_class_name=False).values()
+      one_class_per_module=True)
   for benchmark in all_benchmarks:
     if sys.modules[benchmark.__module__] in _BLACK_LIST_TEST_MODULES:
       continue