MD Settings: Fix About page exception with Advanced lazy loading

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

Index: chrome/browser/resources/settings/settings_page/main_page_behavior.js
diff --git a/chrome/browser/resources/settings/settings_page/main_page_behavior.js b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
index 600499e704e72bcd590b9649e15e087b4fd2290b..3388fb65a89a6e355b7945a52f7653a3c754e7df 100644
--- a/chrome/browser/resources/settings/settings_page/main_page_behavior.js
+++ b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
@@ -149,7 +149,11 @@ var MainPageBehaviorImpl = {
         // 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