v8binding: Handle nullable DOMString return value of OnBeforeUnloadEventHandler

OnBeforeUnloadEventHandler's return type is DOMString?, so we need to
handle the case that the return value is null.  This patch fixes the
behavior in case of null.

Change-Id: I23e4b06174adb42111584f1e84f7ca1c9d04e56c
Reviewed-on: https://chromium-review.googlesource.com/c/1335750Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607984}

third_party/blink/renderer/bindings/core/v8/js_event_handler.cc

@@ -150,11 +150,12 @@ void JSEventHandler::InvokeInternal(EventTarget& event_target,
   //             DOMString.
   if (event.IsBeforeUnloadEvent() &&
       event.type() == event_type_names::kBeforeunload) {
-    DCHECK(result_for_beforeunload);
-    event.preventDefault();
-    BeforeUnloadEvent* before_unload_event = ToBeforeUnloadEvent(&event);
-    if (before_unload_event->returnValue().IsEmpty())
-      before_unload_event->setReturnValue(result_for_beforeunload);
+    if (result_for_beforeunload) {
+      event.preventDefault();
+      BeforeUnloadEvent* before_unload_event = ToBeforeUnloadEvent(&event);
+      if (before_unload_event->returnValue().IsEmpty())
+        before_unload_event->setReturnValue(result_for_beforeunload);
+    }
   } else if (!IsOnBeforeUnloadEventHandler()) {
     if (special_error_event_handling && v8_return_value->IsBoolean() &&
         v8_return_value.As<v8::Boolean>()->Value())