Settings: Fix race condition in MainPageBehavior.

The race condition manifested when transitioning from a subpage to
another sibling subpage, and caused the showing-main-page event to
erroneously be fired after the subpage-expand event.

Bug: 928082
Change-Id: If66e0d0e62cda2d1310dddc067429ca6f7133368
Reviewed-on: https://chromium-review.googlesource.com/c/1459589
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630322}

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

@@ -202,17 +202,21 @@ cr.define('settings', function() {
 
     /**
      * @param {!settings.Route} oldRoute
+     * @return {!Promise<void>}
      * @private
      */
     enterMainPage_: function(oldRoute) {
       const oldSection = this.getSection(oldRoute.section);
       oldSection.classList.remove('expanded');
       this.classList.remove('showing-subpage');
-      requestAnimationFrame(() => {
-        if (settings.lastRouteChangeWasPopstate()) {
-          this.scroller.scrollTop = this.lastScrollTop_;
-        }
-        this.fire('showing-main-page');
+      return new Promise((res, rej) => {
+        requestAnimationFrame(() => {
+          if (settings.lastRouteChangeWasPopstate()) {
+            this.scroller.scrollTop = this.lastScrollTop_;
+          }
+          this.fire('showing-main-page');
+          res();
+        });
       });
     },
 
@@ -318,8 +322,9 @@ cr.define('settings', function() {
           // different sections, but are linked to each other. For example
           // /storage and /accounts (in ChromeOS).
           if (!oldRoute.contains(newRoute) && !newRoute.contains(oldRoute)) {
-            this.enterMainPage_(oldRoute);
-            this.enterSubpage_(newRoute);
+            this.enterMainPage_(oldRoute).then(() => {
+              this.enterSubpage_(newRoute);
+            });
             return;
           }