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}
void RenderFrameImpl::OnVisualStateRequest(uint64_t id) {
  GetRenderWidget()->QueueMessage(
      new FrameHostMsg_VisualStateResponse(routing_id_, id));
}

void RenderFrameImpl::OnVisualStateRequest(uint64_t id) {
  GetRenderWidget()->QueueMessage(
      new FrameHostMsg_VisualStateResponse(routing_id_, id));
}
