Add a DCHECK to ensure that WindowTreeClient has a delegate

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 160 matching lines @@
     : connector_(connector),
       client_id_(0),
       next_window_id_(1),
       next_change_id_(1),
       delegate_(delegate),
       window_manager_delegate_(window_manager_delegate),
       binding_(this),
       tree_(nullptr),
       in_destructor_(false),
       weak_factory_(this) {
+  DCHECK(delegate_);

[tonikitoo, 2017/02/17 17:10:28]

alternatively, we could make 'delegate_' a reference instead, but it would be a
bigger CL.

   // Allow for a null request in tests.
   if (request.is_pending())
     binding_.Bind(std::move(request));
   client::GetTransientWindowClient()->AddObserver(this);
   if (window_manager_delegate)
     window_manager_delegate->SetWindowManagerClient(this);
   if (connector) {  // |connector| can be null in tests.
     gpu_ = ui::Gpu::Create(connector, std::move(io_task_runner));
     compositor_context_factory_ =
         base::MakeUnique<MusContextFactory>(gpu_.get());
@@ skipping 1649 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