[Switch Access] Handle the system menu closing

This change fixes crbug/998959 by setting inSystemMenu_
to false when the system menu (i.e. the right-click
or context menu) is removed from the page.

Bug: 998959
Change-Id: I391b634217513247c4f4215f5a47ffb028886bc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775381Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: Sophie Yang <sophyang@google.com>
Cr-Commit-Position: refs/heads/master@{#691780}

chrome/browser/resources/chromeos/switch_access/navigation_manager.js

@@ -614,6 +614,11 @@ class NavigationManager {
     if (!removedByRWA && treeChange.target !== this.node_)
       return;
 
+    if (this.inSystemMenu_) {
+      // The node removed was the system menu.
+      this.inSystemMenu_ = false;
+    }
+
     this.focusRingManager.clearAll();
 
     // Current node not invalid until after treeChange callback, so move to