Add a JavaScript dialog tab helper.

chrome/browser/extensions/content_script_apitest.cc

Index: chrome/browser/extensions/content_script_apitest.cc
diff --git a/chrome/browser/extensions/content_script_apitest.cc b/chrome/browser/extensions/content_script_apitest.cc
index 126beb51e6cb024bf68142397e4e663a40688c9c..4615de0ac247ce6edd891f1b8a49fd136b1bdaa5 100644
--- a/chrome/browser/extensions/content_script_apitest.cc
+++ b/chrome/browser/extensions/content_script_apitest.cc
@@ -164,12 +164,11 @@ DialogHelper::DialogHelper(content::WebContents* web_contents)
       client_(nullptr) {
   app_modal::JavaScriptDialogManager* dialog_manager_impl =
       app_modal::JavaScriptDialogManager::GetInstance();
-  dialog_manager_ =
-      web_contents_->GetDelegate()->GetJavaScriptDialogManager(web_contents_);
-  DCHECK_EQ(dialog_manager_impl, dialog_manager_);
-
   client_ = new DialogClient(this);
   dialog_manager_impl->SetExtensionsClient(base::WrapUnique(client_));
+
+  dialog_manager_ =
+      web_contents_->GetDelegate()->GetJavaScriptDialogManager(web_contents_);
 }
 
 DialogHelper::~DialogHelper() {