feedback UI: use native window controls on Mac

chrome/browser/extensions/api/feedback_private/feedback_private_api.cc

Index: chrome/browser/extensions/api/feedback_private/feedback_private_api.cc
diff --git a/chrome/browser/extensions/api/feedback_private/feedback_private_api.cc b/chrome/browser/extensions/api/feedback_private/feedback_private_api.cc
index 620ed9221f756329b33e861330a2653fce4096cb..8e227820595902a7e886fe7365e0b4c3d7d8be97 100644
--- a/chrome/browser/extensions/api/feedback_private/feedback_private_api.cc
+++ b/chrome/browser/extensions/api/feedback_private/feedback_private_api.cc
@@ -130,6 +130,11 @@ void FeedbackPrivateAPI::RequestFeedbackForFlow(
       info.trace_id.reset(new int(manager->RequestTrace()));
     }
     info.flow = flow;
+#if defined(OS_MACOSX)
+    info.use_system_window_frame = true;
+#else
+    info.use_system_window_frame = false;
+#endif
 
     std::unique_ptr<base::ListValue> args =
         feedback_private::OnFeedbackRequested::Create(info);