v8binding: Minor fix of OnBeforeUnloadEventHandler's return value

The return type of OnBeforeUnloadEventHandler is DOMString?, so
we should use V8StringResource<kTreatNullAsNullString> instead
of V8String<kDefaultMode>.

Change-Id: I6df9581cf5dd01ae8215d773c95a3e9dc6a78e08
Reviewed-on: https://chromium-review.googlesource.com/c/1341779Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609219}

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

@@ -135,7 +135,7 @@ void JSEventHandler::InvokeInternal(EventTarget& event_target,
   String result_for_beforeunload;
   if (IsOnBeforeUnloadEventHandler()) {
     // TODO(yukiy): use |NativeValueTraits|.
-    V8StringResource<> native_result(v8_return_value);
+    V8StringResource<kTreatNullAsNullString> native_result(v8_return_value);
 
     // |native_result.Prepare()| throws exception if it fails to convert
     // |native_result| to String.