Settings: Set active global scroll handler at the end of the execution queue.

Currently, the global scroll handler gets set as active immediately when the
chrome://settings route changes. However, scroll-handling elements such as
iron-list may require access to the global scroll handler during a route change.
Fix by setting the global scroll handler active value at the next event loop
instead of immediately.

Bug: 835712
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ie02843a8d556a5b44a87f4ba696cab2eeff23e70
Reviewed-on: https://chromium-review.googlesource.com/1100659Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Patti <patricialor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572457}

chrome/browser/resources/settings/global_scroll_target_behavior.js

@@ -52,12 +52,26 @@ cr.define('settings', function() {
 
     /** @override */
     attached: function() {
+      this.active_ = settings.getCurrentRoute() == this.subpageRoute;
       scrollTargetResolver.promise.then(this._setScrollTarget.bind(this));
     },
 
     /** @param {!settings.Route} route */
     currentRouteChanged: function(route) {
-      this.active_ = route == this.subpageRoute;
+      // Immediately set the scroll target to active when this page is
+      // activated, but wait a task to remove the scroll target when the page is
+      // deactivated. This gives scroll handlers like iron-list a chance to
+      // handle scroll events that are fired as a result of the route changing.
+      // TODO(https://crbug.com/859794): Having this timeout can result some
+      // jumpy behaviour in the scroll handlers. |this.active_| can be set
+      // immediately when this bug is fixed.
+      if (route == this.subpageRoute) {
+        this.active_ = true;
+      } else {
+        setTimeout(() => {
+          this.active_ = false;
+        });
+      }
     },
 
     /**