Update telemetry tests to be agnostic to ScrollTopLeftInterop mode

tools/telemetry/telemetry/internal/actions/scroll.js

 // Copyright (c) 2012 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.
 
 // This file provides the ScrollAction object, which scrolls a page
 // to the bottom or for a specified distance:
 //   1. var action = new __ScrollAction(callback, opt_distance_func)
 //   2. action.start(scroll_options)
 'use strict';
 
 (function() {
   var MAX_SCROLL_LENGTH_TIME_MS = 6250;
 
   function ScrollGestureOptions(opt_options) {
     if (opt_options) {
       this.element_ = opt_options.element;
       this.left_start_ratio_ = opt_options.left_start_ratio;
       this.top_start_ratio_ = opt_options.top_start_ratio;
       this.direction_ = opt_options.direction;
       this.speed_ = opt_options.speed;
       this.gesture_source_type_ = opt_options.gesture_source_type;
     } else {
-      this.element_ = document.body;
+      this.element_ = document.scrollingElement;
       this.left_start_ratio_ = 0.5;
       this.top_start_ratio_ = 0.5;
       this.direction_ = 'down';
       this.speed_ = 800;
       this.gesture_source_type_ = chrome.gpuBenchmarking.DEFAULT_INPUT;
     }
   }
 
   function supportedByBrowser() {
     return !!(window.chrome &&
@@ skipping 104 matching lines @@
     this.endMeasuringHook();
 
     // We're done.
     if (this.callback_)
       this.callback_();
   };
 
   window.__ScrollAction = ScrollAction;
   window.__ScrollAction_SupportedByBrowser = supportedByBrowser;
 })();