MD Settings: Fix endless recursion when exiting CBD dialog.

chrome/browser/resources/settings/settings_page/main_page_behavior.js

 // Copyright 2016 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.
 
 /**
  * Responds to route changes by expanding, collapsing, or scrolling to sections
  * on the page. Expanded sections take up the full height of the container. At
  * most one section should be expanded at any given time.
  * @polymerBehavior MainPageBehavior
  */
@@ skipping 102 matching lines @@
     } else if (currentSection) {
       // Expand the section into a subpage or scroll to it on the main page.
       if (currentRoute.isSubpage())
         promise = this.expandSection_(currentSection);
       else if (scrollToSection)
         currentSection.scrollIntoView();
     } else if (settings.Route.ADVANCED.contains(currentRoute) &&
         // Need to exclude routes that correspond to 'non-sectioned' children of
         // ADVANCED, otherwise tryTransitionToSection_ will recurse endlessly.
         !currentRoute.isNavigableDialog) {
+      assert(currentRoute.section);
       promise = this.$$('#advancedPageTemplate').get();
     }
 
     // When this animation ends, another may be necessary. Call this function
     // again after the promise resolves.
     if (promise)
       promise.then(this.tryTransitionToSection_.bind(this, scrollToSection));
   },
 
   /**
@@ skipping 176 matching lines @@
     return /** @type {?SettingsSectionElement} */(
         this.$$('settings-section[section="' + section + '"]'));
   },
 };
 
 /** @polymerBehavior */
 var MainPageBehavior = [
   settings.RouteObserverBehavior,
   MainPageBehaviorImpl,
 ];