Add check for WebContentsModalDialogManager

This does not fix the issue when showing dialogs in a web contents
that does not have a WebContentsModalDialogManager, but let's us fail
in a controlled manner.

Bug: 999011
Change-Id: I57ab762d9c560b01b76f2550b513c7edfef95bd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779173Reviewed-by: Mike Wittman <wittman@chromium.org>
Commit-Queue: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692304}

components/constrained_window/constrained_window_views.cc

@@ -208,12 +208,12 @@ views::Widget* ShowWebModalDialogWithOverlayViews(
 views::Widget* CreateWebModalDialogViews(views::WidgetDelegate* dialog,
                                          content::WebContents* web_contents) {
   DCHECK_EQ(ui::MODAL_TYPE_CHILD, dialog->GetModalType());
+  web_modal::WebContentsModalDialogManager* manager =
+      web_modal::WebContentsModalDialogManager::FromWebContents(web_contents);
+  CHECK(manager);
   return views::DialogDelegate::CreateDialogWidget(
       dialog, nullptr,
-      web_modal::WebContentsModalDialogManager::FromWebContents(web_contents)
-          ->delegate()
-          ->GetWebContentsModalDialogHost()
-          ->GetHostView());
+      manager->delegate()->GetWebContentsModalDialogHost()->GetHostView());
 }
 
 views::Widget* CreateBrowserModalDialogViews(views::DialogDelegate* dialog,