Workaround for scroll anchoring issue with AutoScrollExtensionSettingsWebUITest.

chrome/browser/ui/webui/extensions/extension_settings_browsertest.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.
 
 // TODO(dbeam): test for loading upacked extensions?
 
 GEN('#include "chrome/browser/ui/webui/extensions/' +
     'extension_settings_browsertest.h"');
 
 // The id of the extension from |InstallGoodExtension|.
@@ skipping 305 matching lines @@
     this.nextStep();
   };
   var checkIsScrolled = function() {
     assertGT(document.body.scrollTop, 0);
     this.nextStep();
   };
   var checkScrolledToTop = function() {
     assertEquals(0, document.body.scrollTop);
     this.nextStep();
   };
-  var scrollToTop = function() {
+  var scrollToOrigin = function() {
+    // Reset both top and left to avoid a scroll anchoring adjustment when the
+    // #dev-controls element is inserted (crbug.com/646977).
     document.body.scrollTop = 0;
+    document.body.scrollLeft = 0;
     this.nextStep();
   };
   // Test that a) autoscroll works on first page load and b) updating the
   // page doesn't result in autoscroll triggering again.
   this.steps = [this.waitForPageLoad,
                 checkHasScrollbar,
                 checkIsScrolled,
-                scrollToTop,
+                scrollToOrigin,
                 this.enableDeveloperMode,
                 checkScrolledToTop,
                 testDone];
   this.nextStep();
 });
 
 function ErrorConsoleExtensionSettingsWebUITest() {}
 
 ErrorConsoleExtensionSettingsWebUITest.prototype = {
   __proto__: ExtensionSettingsWebUITest.prototype,
@@ skipping 166 matching lines @@
 
   /** @override */
   browsePreload: ExtensionSettingsWebUITest.prototype.browsePreload +
       '?options=' + GOOD_EXTENSION_ID,
 };
 
 TEST_F('OptionsDialogExtensionSettingsWebUITest', 'testAccessibility',
        function() {
   this.emptyTestForAccessibility();
 });