Moved guest_view_registry to GuestViewManager and made it an instance map

This change allows for the change towards moving GuestViewManager to components
and implementing an extensions specific GuestViewManager that installs extensions-specific
guest types.

BUG=444869

Review URL: https://codereview.chromium.org/1096623002

Cr-Commit-Position: refs/heads/master@{#325919}
 GuestViewBase* GuestViewBase::FromWebContents(const WebContents* web_contents) {
   WebContentsGuestViewMap* guest_map = webcontents_guestview_map.Pointer();
  auto it = guest_map->find(web_contents);
  return it == guest_map->end() ? nullptr : it->second;
}

 GuestViewBase* GuestViewBase::FromWebContents(const WebContents* web_contents) {
   WebContentsGuestViewMap* guest_map = webcontents_guestview_map.Pointer();
  auto it = guest_map->find(web_contents);
  return it == guest_map->end() ? nullptr : it->second;
}
