Track where WebContents are created in order to better understand issue.

components/constrained_window/constrained_window_views.cc

Index: components/constrained_window/constrained_window_views.cc
diff --git a/components/constrained_window/constrained_window_views.cc b/components/constrained_window/constrained_window_views.cc
index 5ae6cd7776da6ae7eb24433ddda5d07c808fa9bd..2f4cfb76842a0a8df8ca47ede9b9a58ad385f68a 100644
--- a/components/constrained_window/constrained_window_views.cc
+++ b/components/constrained_window/constrained_window_views.cc
@@ -6,6 +6,8 @@
 
 #include <algorithm>
 
+#include "base/debug/alias.h"
+#include "base/debug/stack_trace.h"
 #include "components/constrained_window/constrained_window_views_client.h"
 #include "components/guest_view/browser/guest_view_base.h"
 #include "components/web_modal/web_contents_modal_dialog_host.h"
@@ -146,6 +148,12 @@ views::Widget* ShowWebModalDialogViews(
 
 views::Widget* CreateWebModalDialogViews(views::WidgetDelegate* dialog,
                                          content::WebContents* web_contents) {
+  // Temporary to track down http://crbug.com/538612
+  base::debug::StackTrace contents_stack_trace = web_contents->stack_trace();
+  base::debug::Alias(&contents_stack_trace);
+  CHECK(
+      web_modal::WebContentsModalDialogManager::FromWebContents(web_contents));
+
   DCHECK_EQ(ui::MODAL_TYPE_CHILD, dialog->GetModalType());
   return views::DialogDelegate::CreateDialogWidget(
       dialog, nullptr,