Fix a crash on FileChooserImpl

If a renderer process is compromised, and it calls both of
FileChooserImpl::OpenFileChooser() and EnumerateChosenDirectory() via
Mojo, the browser process could crash because ResetOwner() for
the first FileChooserImpl::proxy_ instance was not called.  We
should check nullness of proxy_ before updating it.

Bug: 941008
Change-Id: Ie0c1895ec46ce78d40594b543e49e43b420af675
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1520509
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640580}
  void WaitForMessage(int32_t* out_routing_id,
                      FrameHostMsg_OpenURL_Params* out_params) {
    run_loop_.Run();
    *out_routing_id = intercepted_routing_id_;
    *out_params = intercepted_params_;
  }

  void WaitForMessage(int32_t* out_routing_id,
                      FrameHostMsg_OpenURL_Params* out_params) {
    run_loop_.Run();
    *out_routing_id = intercepted_routing_id_;
    *out_params = intercepted_params_;
  }
