Add/update checks to diagnose the NPE issue

ash/display/display_controller.cc

Index: ash/display/display_controller.cc
diff --git a/ash/display/display_controller.cc b/ash/display/display_controller.cc
index b5b70c3f987bd21bc984f27f5094cb9844940bc0..ca213cf834e6e24b4659feb85b9a03de01956448 100644
--- a/ash/display/display_controller.cc
+++ b/ash/display/display_controller.cc
@@ -630,10 +630,12 @@ void DisplayController::OnDisplayRemoved(const gfx::Display& display) {
       primary_tree_host_for_replace_ = host_to_delete;
       return;
     }
-    DCHECK_EQ(1U, window_tree_hosts_.size());
     primary_display_id = ScreenUtil::GetSecondaryDisplay().id();
-    AshWindowTreeHost* primary_host = host_to_delete;
+    CHECK_NE(gfx::Display::kInvalidDisplayID, primary_display_id);
+
+    CHECK_EQ(1U, window_tree_hosts_.count(primary_display_id));
 
+    AshWindowTreeHost* primary_host = host_to_delete;
     // Delete the other host instead.
     host_to_delete = window_tree_hosts_[primary_display_id];
     GetRootWindowSettings(GetWindow(host_to_delete))->display_id = display.id();