Main frame's unique name can always be null.

content/browser/frame_host/frame_tree_node.cc

Index: content/browser/frame_host/frame_tree_node.cc
diff --git a/content/browser/frame_host/frame_tree_node.cc b/content/browser/frame_host/frame_tree_node.cc
index 8af110a789dc77d5e28b7e38653f807a237a2dbb..9d6eac59956f040c18ba9478aa7efcc9d024b4c9 100644
--- a/content/browser/frame_host/frame_tree_node.cc
+++ b/content/browser/frame_host/frame_tree_node.cc
@@ -232,6 +232,15 @@ void FrameTreeNode::SetFrameName(const std::string& name,
     DCHECK_EQ(unique_name, replication_state_.unique_name);
     return;
   }
+
+  if (parent()) {
+    // Non-main frames should have a non-empty unique name.
+    DCHECK(!unique_name.empty());
+  } else {
+    // Unique name of main frames should always stay empty.
+    DCHECK(unique_name.empty());

[Charlie Reis, 2016/09/15 20:04:12]

This is in response to an IPC, right?  Should we enforce this in any way for
release builds, like clearing the unique_name if it isn't empty for a main
frame?

[Łukasz Anforowicz, 2016/09/15 20:55:13]

Usually yes, but not necessarily.  For example - a non-IPC call can be made from
the place I had to fix-up in WebContentsImpl::Init.
I am not sure.  I think doing this for emptiness of main frame is not needed:

1. Browser should not look at the unique name of the main frame.

2. Old code worked fine when unique name might have been non-empty.

3. The DCHECKs I've added are mainly to help me^H^H^H future maintainers avoid
the mistake I've made in patchset #1 (where I've modified the unique name format
in Blink, but forgot about this one place in browser-side).


I guess some browser-side validation might still be desirable - most importantly
to make sure the unique names are truly unique (AFAIR we had a security bug
where duplicate unique names where one part of a bigger repro/puzzle).  Still -
even non-malicious renderers can hit a race and generate a non-unique name, so I
don't see how to do the enforcement here (we can't kill a renderer, but there is
no clear fallback).

+  }
+
   RecordUniqueNameLength(unique_name.size());
   render_manager_.OnDidUpdateName(name, unique_name);
   replication_state_.name = name;