Output warning when the container window doesn't exist unexpectedly

ui/keyboard/keyboard_controller.cc

Index: ui/keyboard/keyboard_controller.cc
diff --git a/ui/keyboard/keyboard_controller.cc b/ui/keyboard/keyboard_controller.cc
index e0912a72d1b8c9fdec50c2ded0d21086bd688b3e..5e1680d60722949359f3642c720c14bdacda3386 100644
--- a/ui/keyboard/keyboard_controller.cc
+++ b/ui/keyboard/keyboard_controller.cc
@@ -410,8 +410,9 @@ void KeyboardController::OnShowImeIfNeeded() {
 }
 
 void KeyboardController::ShowKeyboardInternal(int64_t display_id) {
-  if (!container_.get())
-    return;
+  // The container window should have been created already when
+  // |Shell::CreateKeyboard| is called.
+  DCHECK(container_.get());
 
   if (container_->children().empty()) {
     keyboard::MarkKeyboardLoadStarted();