[ntp] Unregister handler as listener of file select dialog in destructor

Otherwise, listener methods can be called on the destroyed new tab page
handler object, which could lead to crashes.

Bug: 1144317
Change-Id: Ibfc50c3e903377494be804dac2b05fdb57e3344d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522129
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824984}

chrome/browser/ui/webui/new_tab_page/new_tab_page_handler.cc

@@ -403,6 +403,9 @@ NewTabPageHandler::~NewTabPageHandler() {
   for (auto bitmap_request_id : bitmap_request_ids_) {
     bitmap_fetcher_service_->CancelRequest(bitmap_request_id);
   }
+  if (select_file_dialog_) {
+    select_file_dialog_->ListenerDestroyed();
+  }
 }
 
 // static