Add a histogram to measure the number of idle sockets when a TCP connection is established.

net/base/client_socket_pool.h

Index: net/base/client_socket_pool.h
diff --git a/net/base/client_socket_pool.h b/net/base/client_socket_pool.h
index dd08c24bdd6dd0bc3092b750ee6dae26c827d552..6c92f37ac0d6c35f9ba2213eb727469c1a8b51e9 100644
--- a/net/base/client_socket_pool.h
+++ b/net/base/client_socket_pool.h
@@ -73,6 +73,9 @@ class ClientSocketPool : public base::RefCounted<ClientSocketPool> {
     return idle_socket_count_;
   }
 
+  // The total number of idle sockets in a connection group.
+  int IdleSocketCountInGroup(const std::string& group_name) const;
+
  private:
   friend class base::RefCounted<ClientSocketPool>;