Add/update checks to diagnose the NPE issue

BUG=426292

Review URL: https://codereview.chromium.org/803693002

Cr-Commit-Position: refs/heads/master@{#308208}

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();