MD Settings: Fix About page exception with Advanced lazy loading

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 131 matching lines @@
       // 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();
+
+      // The About page also uses this behavior, and doesn't expand Advanced.

[dpapad, 2017/03/23 23:09:58]

Ah, I understand now the problem. The code assumes that the only usage of
MainPageBehavior is from <settings-basic-page>, but <settings-about-page> also
uses it. Can we make the code more explicit as follows (or do you think it is
gross)?

if (this.tagName == 'SETTINGS-BASIC-PAGE') {
  // Force the advanced page to be loaded and rendered.
  promise = this.$$('#advancedPageTemplate').get();
}

The advantage of this is that it will NOT silently fail, if for some reason
|this| is <settings-basic-page> but the advancedPageTemplate is not found.

[tommycli, 2017/03/24 19:11:39]

Done.

+      var advancedTemplate = this.$$('#advancedPageTemplate');
+      if (advancedTemplate)
+        promise = advancedTemplate.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));
   },
 
   /**
    * If the current animation is inconsistent with the current route, stops the
@@ skipping 175 matching lines @@
     return /** @type {?SettingsSectionElement} */(
         this.$$('settings-section[section="' + section + '"]'));
   },
 };
 
 /** @polymerBehavior */
 var MainPageBehavior = [
   settings.RouteObserverBehavior,
   MainPageBehaviorImpl,
 ];