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

tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/__init__.py

Index: tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/__init__.py
diff --git a/tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/__init__.py b/tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/__init__.py
index 799c2462114f876d084458af5fea9785446251c1..c5699ad3ace02bfb388353e41692552c05488196 100644
--- a/tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/__init__.py
+++ b/tools/chrome_proxy/integration_tests/chrome_proxy_pagesets/__init__.py
@@ -6,8 +6,9 @@ import inspect
 import os
 import sys
 
-from telemetry.core import discover
 from telemetry import story
+from telemetry.util import classes_util
+
 import video
 
 
@@ -15,8 +16,7 @@ import video
 start_dir = os.path.dirname(os.path.abspath(__file__))
 top_level_dir = os.path.abspath(os.path.join(start_dir, os.pardir, os.pardir))
 base_class = story.StorySet
-for cls in discover.DiscoverClasses(
-    start_dir, top_level_dir, base_class).values():
+for cls in classes_util.DiscoverClasses(start_dir, top_level_dir, base_class):
   setattr(sys.modules[__name__], cls.__name__, cls)
 
 # DiscoverClasses makes the assumption that there is exactly one matching