Fix broken guest NTP

This CL fixes the regular guest NTP where it seems to be unreachable.

Bug: 1144597
Change-Id: Ie1d5a8ca5f059e15b3c2526b01e67ad4f12aebee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512889Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Mariam Ali <alimariam@google.com>
Cr-Commit-Position: refs/heads/master@{#823665}

chrome/browser/ui/webui/ntp/ntp_resource_cache.cc

@@ -222,7 +222,7 @@ base::RefCountedMemory* NTPResourceCache::GetNewTabGuestHTML() {
       GuestNTPInfo guest_ntp_info{kLearnMoreGuestSessionUrl, IDR_GUEST_TAB_HTML,
                                   IDS_NEW_TAB_GUEST_SESSION_HEADING,
                                   IDS_NEW_TAB_GUEST_SESSION_DESCRIPTION};
-      CreateNewTabGuestHTML(guest_ntp_info);
+      new_tab_guest_html_ = CreateNewTabGuestHTML(guest_ntp_info);
     }
 
     return new_tab_guest_html_.get();