Repro:
1. Patch https://chromium-review.googlesource.com/c/chromium/src/+/959613
2. Build an run chrome in mash mode
3. Click "Power/Shutdown" icon in system tray bubble that creates
   a widget to embed a WebContents using
4. Repeat step 3 to create a 2nd window;
5. Close either of the two embedder windows

Crash happens at step 5. Looks like window server try to focus the embedded window,
whose root window is the window for embed roots and does not have focus client.
====
[59553:59553:0313/143532.469293:FATAL:focus_synchronizer.cc(46)] Check failed: client::GetFocusClient(root).                                                                        
#0 0x7f80e7388a0d base::debug::StackTrace::StackTrace()           
#1 0x7f80e738703c base::debug::StackTrace::StackTrace()
#2 0x7f80e74083fd logging::LogMessage::~LogMessage()
#3 0x7f80dc0c054b aura::FocusSynchronizer::SetFocusFromServer()
#4 0x7f80dc112bd1 aura::WindowTreeClient::OnWindowFocused() 
#5 0x7f80dc2601f0 ui::mojom::WindowTreeClientStubDispatch::Accept() 
#6 0x7f80dc12ce43 ui::mojom::WindowTreeClientStub<>::Accept() 
#7 0x7f80e58fed28 mojo::InterfaceEndpointClient::HandleValidatedMessage() 
#8 0x7f80e58fd801 mojo::InterfaceEndpointClient::HandleIncomingMessageThunk::Accept()