Add an opt-out header for HTTP throttling. Never throttle for localhost.

net/base/net_util.h

Index: net/base/net_util.h
diff --git a/net/base/net_util.h b/net/base/net_util.h
index 9e30d2b052948701a24fda85c21908ef503d271a..288a5b122c7af5557130ba7c4065e6550489a934 100644
--- a/net/base/net_util.h
+++ b/net/base/net_util.h
@@ -427,6 +427,14 @@ const uint16* GetPortFieldFromSockaddr(const struct sockaddr* address,
 int GetPortFromSockaddr(const struct sockaddr* address,
                         socklen_t address_len);
 
+// Returns true if |host| is "localhost" or one of the IP-address
+// representations of the loopback address (IPv4 127.0.0.0/8 or IPv6 ::1).
+//
+// Note that this function does not check for IP addresses other than
+// the above being mapped back to the local machine, it only checks
+// for those exact IP addresses.
+bool IsLocalhost(const std::string& host);
+
 // struct that is used by GetNetworkList() to represent a network
 // interface.
 struct NetworkInterface {