Revert of Discover should return a list of classes, not a dictionary. (patchset ...

Revert of Discover should return a list of classes, not a dictionary. (patchset #4 id:120001 of https://codereview.chromium.org/1194723002/)

Reason for revert:
Appears to break benchmarks.benchmark_smoke_unittest.BenchmarkSmokeTest.startup.cold.blank_page

[1/1] benchmarks.benchmark_smoke_unittest.BenchmarkSmokeTest.startup.cold.blank_page failed unexpectedly 0.1250s:
  Pages: []
  RESULT telemetry_page_measurement_results: num_failed= 1 count
  RESULT telemetry_page_measurement_results: num_errored= 0 count

  View result at file://E:\b\build\slave\XP_Tests__1_\build\src\tools\telemetry\telemetry\unittest_util\results.html
  Benchmark startup.cold.blank_page run takes 0 seconds

  Traceback (most recent call last):
    Run at tools\telemetry\telemetry\benchmark.py:194
      max_failures=self._max_failures)
    Run at tools\telemetry\telemetry\internal\story_runner.py:216
      expectations, user_story, results, state)
    _RunUserStoryAndProcessErrorIfNeeded at tools\telemetry\telemetry\internal\story_runner.py:78
      state.WillRunUserStory(user_story)
    WillRunUserStory at tools\telemetry\telemetry\page\shared_page_state.py:161
      self.browser = self._possible_browser.Create(self._finder_options)
    Create at tools\telemetry\telemetry\internal\backends\chrome\desktop_browser_finder.py:67
      browser_backend, self._platform_backend, self._credentials_path)
    __init__ at tools\telemetry\telemetry\core\browser.py:47
      self._browser_backend.profile_directory)
    FlushSystemCacheForDirectory at tools\telemetry\telemetry\core\platform\__init__.py:159
      return self._platform_backend.FlushSystemCacheForDirectory(directory)
    FlushSystemCacheForDirectory at tools\telemetry\telemetry\core\platform\desktop_platform_backend.py:23
      assert flush_command, 'You must build clear_system_cache first'
  AssertionError: You must build clear_system_cache first

  Locals:
    directory     : 'c:\\docume~1\\chrome~2\\locals~1\\temp\\tmpmxy3b0'
    flush_command : None

  Traceback (most recent call last):
    File "E:\b\build\slave\XP_Tests__1_\build\src\tools\telemetry\telemetry\decorators.py", line 118, in wrapper
      func(*args, **kwargs)
    File "E:\b\build\slave\XP_Tests__1_\build\src\tools\perf\benchmarks\benchmark_smoke_unittest.py", line 75, in BenchmarkSmokeTest
      msg='Failed: %s' % benchmark)
  AssertionError: Failed: <class 'benchmarks.startup.StartupColdBlankPage'>

https://build.chromium.org/p/chromium.win/builders/XP%20Tests%20%281%29/builds/38411

Original issue's description:
> Discover should return a list of classes, not a dictionary.
> Add classes_util.
>
> BUG=498968
> CQ_EXTRA_TRYBOTS=tryserver.chromium.perf:linux_perf_bisect;tryserver.chromium.perf:mac_perf_bisect;tryserver.chromium.perf:win_perf_bisect;tryserver.chromium.perf:android_nexus5_perf_bisect
>
> Committed: https://crrev.com/57fa7b351455df84db4507f370417c4808101e99
> Cr-Commit-Position: refs/heads/master@{#336283}

TBR=dtu@chromium.org,nednguyen@google.com,kbr@chromium.org,bengr@chromium.org,aiolos@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=498968

Review URL: https://codereview.chromium.org/1216433004

Cr-Commit-Position: refs/heads/master@{#336320}