[Perf] Update references to telemetry.core.discover

tools/perf/page_sets/infinite_scroll_cases.py

 # Copyright 2017 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.
 
 import sys
 
 from page_sets.login_helpers import facebook_login
 from page_sets.system_health import platforms
-from telemetry.core import discover
 from telemetry.page import page
 from telemetry.page import shared_page_state
 from telemetry import story
 
+from py_utils import discover
+
 class _InfiniteScrollStory(page.Page):
   """ Base class for infinite scroll stories."""
 
   NAME = NotImplemented
   URL = NotImplemented
   SUPPORTED_PLATFORMS = platforms.ALL_PLATFORMS
 
   SCROLL_DISTANCE = 25000
   SCROLL_STEP = 1000
   MAX_SCROLL_RETRIES = 5
@@ skipping 124 matching lines @@
       self.AddStory(story_class(self))
 
 def _FindInfiniteScrollStoryClasses(platform):
   # Sort the classes by their names so that their order is stable and
   # deterministic.
   for unused_cls_name, cls in sorted(discover.DiscoverClassesInModule(
       module=sys.modules[__name__], base_class=_InfiniteScrollStory,
       index_by_class_name=True).iteritems()):
     if platform in cls.SUPPORTED_PLATFORMS:
       yield cls