Makes ScreenMus not check that displays are non-empty

ui/views/mus/screen_mus.cc

 // Copyright 2016 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/views/mus/screen_mus.h"
 
 #include "mojo/converters/geometry/geometry_type_converters.h"
 #include "services/shell/public/cpp/connection.h"
 #include "services/shell/public/cpp/connector.h"
 #include "ui/gfx/display_finder.h"
@@ skipping 80 matching lines @@
 
   display_manager_->AddObserver(
       display_manager_observer_binding_.CreateInterfacePtrAndBind());
 
   // We need the set of displays before we can continue. Wait for it.
   //
   // TODO(rockot): Do something better here. This should not have to block tasks
   // from running on the calling thread. http://crbug.com/594852.
   display_manager_observer_binding_.WaitForIncomingMethodCall();
 
-  // The WaitForIncomingMethodCall() should have supplied the set of Displays.
-  DCHECK(displays_.size());
+  // The WaitForIncomingMethodCall() should have supplied the set of Displays,
+  // unless mus is going down, in which case encountered_error() is true.
+  if (displays_.empty()) {
+    DCHECK(display_manager_.encountered_error());
+    // In this case we install a default display and assume the process is
+    // going to exit shortly so that the real value doesn't matter.
+    displays_.push_back(gfx::Display(0xFFFFFFFF, gfx::Rect(0, 0, 801, 802)));
+  }
 }
 
 int ScreenMus::FindDisplayIndexById(int64_t id) const {
   for (size_t i = 0; i < displays_.size(); ++i) {
     if (displays_[i].id() == id)
       return static_cast<int>(i);
   }
   return -1;
 }
 
@@ skipping 87 matching lines @@
 
 void ScreenMus::RemoveObserver(gfx::DisplayObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
 void ScreenMus::OnDisplays(mojo::Array<mus::mojom::DisplayPtr> displays) {
   // This should only be called once from Init() before any observers have been
   // added.
   DCHECK(displays_.empty());
   displays_ = displays.To<std::vector<gfx::Display>>();
+  DCHECK(!displays_.empty());
   for (size_t i = 0; i < displays.size(); ++i) {
     if (displays[i]->is_primary) {
       primary_display_index_ = static_cast<int>(i);
       // TODO(sky): Make WindowManagerFrameValues per display.
       WindowManagerFrameValues frame_values =
           displays[i]->frame_decoration_values.To<WindowManagerFrameValues>();
       WindowManagerFrameValues::SetInstance(frame_values);
     }
   }
 }
@@ skipping 18 matching lines @@
   DCHECK_NE(-1, index);
   // Another display must become primary before the existing primary is
   // removed.
   DCHECK_NE(index, primary_display_index_);
   const gfx::Display display = displays_[index];
   FOR_EACH_OBSERVER(gfx::DisplayObserver, observers_,
                     OnDisplayRemoved(display));
 }
 
 }  // namespace views