Restrict transmission of external exp ids to signed in users.

components/ntp_snippets/remote/ntp_snippets_fetcher.cc

Index: components/ntp_snippets/remote/ntp_snippets_fetcher.cc
diff --git a/components/ntp_snippets/remote/ntp_snippets_fetcher.cc b/components/ntp_snippets/remote/ntp_snippets_fetcher.cc
index 62236a8a375cab50981b1a7798b0ce55f963798c..09b7d6ee8ad6d0c54ee614106497d5975b1a72dc 100644
--- a/components/ntp_snippets/remote/ntp_snippets_fetcher.cc
+++ b/components/ntp_snippets/remote/ntp_snippets_fetcher.cc
@@ -681,6 +681,7 @@ std::string NTPSnippetsFetcher::RequestBuilder::BuildHeaders() const {
   variations::AppendVariationHeaders(url_,
                                      false,  // incognito
                                      false,  // uma_enabled
+                                     false,  // is_signed_in

[Marc Treib, 2016/12/09 12:34:31]

Essentially the same comment that mathp had: Can you please add a comment
explaining why it's okay to simply set this to false here? I was also concerned
that this would stop variations from working.

[Alexei Svitkine (slow), 2016/12/09 15:53:58]

Done.

                                      &headers);
   return headers.ToString();
 }