Main frame's unique name can always be null.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 6aabf786c74fc3c2979ebaf2c4bcde724c3023cb..f410d1ae4ff5cdc551e5ad4d72803091b1361eb3 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -1538,9 +1538,9 @@ void WebContentsImpl::Init(const WebContents::CreateParams& params) {
                            params.main_frame_routing_id,
                            main_frame_widget_routing_id);
 
-  // blink::FrameTree::setName uses |name| as the |unique_name| for the main
-  // frame - let's do the same thing here.
-  std::string unique_name = params.main_frame_name;
+  // blink::FrameTree::setName always keeps |unique_name| empty in case of a
+  // main frame - let's do the same thing here.
+  std::string unique_name = "";

[dcheng, 2016/09/15 06:33:23]

Omit "" and just default initialize (it's slightly more efficient)

[Łukasz Anforowicz, 2016/09/15 15:55:18]

Done.

   frame_tree_.root()->SetFrameName(params.main_frame_name, unique_name);
 
   WebContentsViewDelegate* delegate =