Reland of Makes ScreenMus not check that displays are non-empty

ui/views/mus/screen_mus.cc

Index: ui/views/mus/screen_mus.cc
diff --git a/ui/views/mus/screen_mus.cc b/ui/views/mus/screen_mus.cc
index faa31ad3463dc1ac9b46a703fc11236c1faf2e7b..1a66fa864d6bcfea27c0062a17de6d4c488da384 100644
--- a/ui/views/mus/screen_mus.cc
+++ b/ui/views/mus/screen_mus.cc
@@ -98,8 +98,14 @@
   // 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 {
@@ -207,6 +213,7 @@
   // 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);