Fix crashes in RenderFrameImpl::OnSelectPopupMenuItem(s)

ExternalPopupMenu::DidSelectItem(s) can delete the RenderFrameImpl.
We need to reset external_popup_menu_ before calling it.

Bug: 912211
Change-Id: Ia9a628e144464a2ebb14ab77d3a693fd5cead6fc
Reviewed-on: https://chromium-review.googlesource.com/c/1381325
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618026}
RenderFrameImpl* RenderFrameImpl::FromRoutingID(int routing_id) {
  auto iter = g_routing_id_frame_map.Get().find(routing_id);
  if (iter != g_routing_id_frame_map.Get().end())
    return iter->second;
  return nullptr;
}

RenderFrameImpl* RenderFrameImpl::FromRoutingID(int routing_id) {
  auto iter = g_routing_id_frame_map.Get().find(routing_id);
  if (iter != g_routing_id_frame_map.Get().end())
    return iter->second;
  return nullptr;
}
