Mac: Add CHECK() in ConstrainedWindowMac::GetDialogManager().

chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm

 // Copyright (c) 2012 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 "chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/logging.h"
@@ skipping 63 matching lines @@
     delegate_->OnConstrainedWindowClosed(this);
 }
 
 bool ConstrainedWindowMac::DialogWasShown() {
   // If the dialog was shown, |native_manager_| would have been released.
   return !native_manager_;
 }
 
 WebContentsModalDialogManager* ConstrainedWindowMac::GetDialogManager() {
   DCHECK(web_contents_);
-  return WebContentsModalDialogManager::FromWebContents(web_contents_);
+  WebContentsModalDialogManager* dialog_manager =
+      WebContentsModalDialogManager::FromWebContents(web_contents_);
+  // If WebContentsModalDialogManager::CreateForWebContents(web_contents_) was
+  // never called, then the manager will be null. E.g., for browser tabs,
+  // TabHelpers::AttachTabHelpers() calls CreateForWebContents(). It is invalid
+  // to show a dialog on some kinds of WebContents. Crash cleanly in that case.
+  CHECK(dialog_manager);
+  return dialog_manager;
 }