Update telemetry tests to be agnostic to ScrollTopLeftInterop mode

tools/telemetry/telemetry/internal/actions/scroll_bounce.py

 # Copyright 2014 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 os
 
 from telemetry.internal.actions import page_action
 
 
 class ScrollBounceAction(page_action.PageAction):
   def __init__(self, selector=None, text=None, element_function=None,
@@ skipping 20 matching lines @@
     self._overscroll = overscroll
     # It's the transitions we really want to stress, make this big.
     self._repeat_count = repeat_count
     # 7 pixels per frame should be plenty of frames.
     self._speed = speed_in_pixels_per_second
     self._synthetic_gesture_source = ('chrome.gpuBenchmarking.%s_INPUT' %
                                       synthetic_gesture_source)
 
     if (self._selector is None and self._text is None and
         self._element_function is None):
-      self._element_function = 'document.body'
+      self._element_function = 'document.scrollingElement'
 
   def WillRunAction(self, tab):
     for js_file in ['gesture_common.js', 'scroll_bounce.js']:
       with open(os.path.join(os.path.dirname(__file__), js_file)) as f:
         js = f.read()
         tab.ExecuteJavaScript(js)
 
     # Fail if browser doesn't support synthetic scroll bounce gestures.
     if not tab.EvaluateJavaScript(
         'window.__ScrollBounceAction_SupportedByBrowser()'):
@@ skipping 37 matching lines @@
                 self._top_start_ratio,
                 self._direction,
                 self._distance,
                 self._overscroll,
                 self._repeat_count,
                 self._speed)
     page_action.EvaluateCallbackWithElement(
         tab, code, selector=self._selector, text=self._text,
         element_function=self._element_function)
     tab.WaitForJavaScriptExpression('window.__scrollBounceActionDone', 60)