MD Settings: Fix Back navigation from section route to BASIC route.

chrome/test/data/webui/settings/basic_page_browsertest.js

 // Copyright 2015 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.
 
 /** @fileoverview Suite of tests for the Settings basic page. */
 
 GEN_INCLUDE(['settings_page_browsertest.js']);
 
 /**
  * @constructor
@@ skipping 34 matching lines @@
       else
         sections = sections.concat(['internet', 'device']);
 
       for (var i = 0; i < sections.length; i++) {
         var section = self.getSection(page, sections[i]);
         expectTrue(!!section);
         self.verifySubpagesHidden(section);
       }
     });
 
-    // This test checks for a regression that occurred with scrollToSection_
-    // failing to find its host element.
     test('scroll to section', function() {
       // Setting the page and section will cause a scrollToSection_.
       settings.navigateTo(settings.Route.ON_STARTUP);
 
+      var page = self.getPage('basic');
+
       return new Promise(function(resolve, reject) {
+        // This test checks for a regression that occurred with scrollToSection_
+        // failing to find its host element.
         var intervalId = window.setInterval(function() {
           var page = self.getPage('basic');
           if (self.getSection(page, settings.getCurrentRoute().section)) {
             window.clearInterval(intervalId);
             resolve();
           }
         }, 55);
-      }.bind(self));
+      }.bind(self)).then(function() {
+        // Should be scrolled to the On Startup section.
+        assertNotEquals(0, page.scroller.scrollTop);
+
+        return new Promise(function(resolve) {
+          listenOnce(window, 'popstate', resolve);
+          settings.navigateToPreviousRoute();
+        });
+      }).then(function() {
+        // Should be at the top of the page after going Back from the section.
+        assertEquals(0, page.scroller.scrollTop);
+      });
     });
   });
 
   // Run all registered tests.
   mocha.run();
 });