Remove the ability of webpages to specify strings for the onbeforeunload dialog.

components/test_runner/web_frame_test_proxy.h

Index: components/test_runner/web_frame_test_proxy.h
diff --git a/components/test_runner/web_frame_test_proxy.h b/components/test_runner/web_frame_test_proxy.h
index 363f5e3794ec16e9fa2b2b9c532d139f5ce06d40..1cbe11f80ef6c58c6c714f658997e1a3524bb30e 100644
--- a/components/test_runner/web_frame_test_proxy.h
+++ b/components/test_runner/web_frame_test_proxy.h
@@ -167,10 +167,9 @@ class WebFrameTestProxy : public Base {
     return true;
   }
 
-  bool runModalBeforeUnloadDialog(bool is_reload,
-                                  const blink::WebString& message) override {
+  bool runModalBeforeUnloadDialog(bool is_reload) override {
     base_proxy_->GetDelegate()->PrintMessage(
-        std::string("CONFIRM NAVIGATION: ") + message.utf8().data() + "\n");
+        std::string("CONFIRM NAVIGATION\n"));
     return !base_proxy_->GetInterfaces()
                 ->TestRunner()
                 ->ShouldStayOnPageAfterHandlingBeforeUnload();