Have WindowTreeClient detect error state earlier

ui/aura/mus/window_tree_client.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura/mus/window_tree_client.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 165 matching lines @@
     Env::GetInstance()->set_context_factory(compositor_context_factory_.get());
   }
 }
 
 WindowTreeClient::~WindowTreeClient() {
   in_destructor_ = true;
 
   for (WindowTreeClientObserver& observer : observers_)
     observer.OnWillDestroyClient(this);
 
-  std::vector<Window*> non_owned;
-  WindowTracker tracker;
-  while (!windows_.empty()) {
-    IdToWindowMap::iterator it = windows_.begin();
-    if (WasCreatedByThisClient(it->second)) {
-      const Id window_id = it->second->server_id();
-      delete it->second->GetWindow();
-      DCHECK_EQ(0u, windows_.count(window_id));
-    } else {
-      tracker.Add(it->second->GetWindow());
-      windows_.erase(it);
-    }
-  }
-
-  // Delete the non-owned windows last. In the typical case these are roots. The
-  // exception is the window manager and embed roots, which may know about
-  // other random windows that it doesn't own.
-  while (!tracker.windows().empty())
-    delete tracker.windows().front();
+  // Clients should properly delete all of their windows before shutdown.
+  DCHECK(windows_.empty());

[sky, 2017/02/13 23:16:15]

I'm ok with making this a CHECK as I suspect this is going to come up, and it
would be good to know the reason vs mysterious errors down the line.

[jonross, 2017/02/14 20:35:21]

Done.

 
   for (WindowTreeClientObserver& observer : observers_)

[sky, 2017/02/13 23:16:15]

At this point we don't need be OnWillDestroyClient and OnDidDestroyClient. Maybe
remove the OnDidDestroyClient? It looks like the current user of it can
trivially be converted.

[jonross, 2017/02/14 20:35:21]

Done.

     observer.OnDidDestroyClient(this);
 
   capture_synchronizer_.reset();
 
   client::GetTransientWindowClient()->RemoveObserver(this);
 }
 
 void WindowTreeClient::ConnectViaWindowTreeFactory() {
   // The client id doesn't really matter, we use 101 purely for debugging.
   client_id_ = 101;
@@ skipping 1601 matching lines @@
   return ScheduleInFlightChange(base::MakeUnique<InFlightCaptureChange>(
       this, capture_synchronizer_.get(), window));
 }
 
 uint32_t WindowTreeClient::CreateChangeIdForFocus(WindowMus* window) {
   return ScheduleInFlightChange(base::MakeUnique<InFlightFocusChange>(
       this, focus_synchronizer_.get(), window));
 }
 
 }  // namespace aura