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

net/base/client_socket_pool.cc

Index: net/base/client_socket_pool.cc
diff --git a/net/base/client_socket_pool.cc b/net/base/client_socket_pool.cc
index 651fbe275a75449cb09d23f8ed7a80f1649e4207..4171b0c3d07de2d76f4819675ec0c00cf7bf5a2a 100644
--- a/net/base/client_socket_pool.cc
+++ b/net/base/client_socket_pool.cc
@@ -122,6 +122,14 @@ void ClientSocketPool::CloseIdleSockets() {
   CleanupIdleSockets(true);
 }
 
+int ClientSocketPool::IdleSocketCountInGroup(
+    const std::string& group_name) const {
+  GroupMap::const_iterator i = group_map_.find(group_name);
+  DCHECK(i != group_map_.end());
+
+  return i->second.idle_sockets.size();
+}
+
 bool ClientSocketPool::IdleSocket::ShouldCleanup(base::TimeTicks now) const {
   bool timed_out = (now - start_time) >=
       base::TimeDelta::FromSeconds(kIdleTimeout);