Thanks for reverting. Looks like this is uncovering an existing bug in Scroll action implementation: 

TimeoutException: Timeout after 60s while waiting for JavaScript:window.__scrollActionDone


Traceback (most recent call last):
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/story_runner.py", line 126, in _RunStoryAndProcessErrorIfNeeded
      state.RunStory(results)
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/page/shared_page_state.py", line 300, in RunStory
      self._current_page.Run(self)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/page/__init__.py", line 99, in Run
      self.RunPageInteractions(action_runner)
    File "/b/swarming/w/ir/tools/perf/page_sets/system_health/system_health_story.py", line 123, in RunPageInteractions
      self._DidLoadDocument(action_runner)
    File "/b/swarming/w/ir/tools/perf/page_sets/system_health/browsing_stories.py", line 996, in _DidLoadDocument
      self._Scroll(action_runner, self.SCROLL_DISTANCE, self.SCROLL_STEP)
    File "/b/swarming/w/ir/tools/perf/page_sets/system_health/browsing_stories.py", line 1011, in _Scroll
      action_runner.ScrollPage(distance=min(remaining, step_size) + 1)
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/actions/action_runner.py", line 411, in ScrollPage
      use_touch=use_touch, synthetic_gesture_source=synthetic_gesture_source))
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/actions/action_runner.py", line 62, in _RunAction
      action.RunAction(self._tab)
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/actions/scroll.py", line 116, in RunAction
      tab.WaitForJavaScriptCondition('window.__scrollActionDone', timeout=60)
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/browser/web_contents.py", line 245, in WaitForJavaScriptCondition
      return self._inspector_backend.WaitForJavaScriptCondition(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py", line 307, in WaitForJavaScriptCondition
      self.GetCurrentConsoleOutputBuffer())
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py", line 41, in Inner
      inspector_backend._ConvertExceptionFromInspectorWebsocket(e)
    File "/b/swarming/w/ir/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 75, in traced_function
      return func(*args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py", line 38, in Inner
      return func(inspector_backend, *args, **kwargs)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_backend.py", line 196, in GetCurrentConsoleOutputBuffer
      return self._console.GetCurrentConsoleOutputBuffer(timeout)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_console.py", line 38, in GetCurrentConsoleOutputBuffer
      self._inspector_websocket.DispatchNotifications(timeout)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_websocket.py", line 156, in DispatchNotifications
      self._Receive(timeout)
    File "/b/swarming/w/ir/third_party/catapult/telemetry/telemetry/internal/backends/chrome_inspector/inspector_websocket.py", line 181, in _Receive
      raise WebSocketException(err)
  TimeoutException: Timeout after 60s while waiting for JavaScript:window.__scrollActionDone