Remove unused HostResolver::Observer.

net/dns/async_host_resolver.h

Index: net/dns/async_host_resolver.h
diff --git a/net/dns/async_host_resolver.h b/net/dns/async_host_resolver.h
index 9ad5c01c3e5b388cd968db1b7e60a25dd5750c1f..432983defa97e27a22eaa3b2e759c08bf39541de 100644
--- a/net/dns/async_host_resolver.h
+++ b/net/dns/async_host_resolver.h
@@ -9,7 +9,6 @@
 #include <list>
 #include <map>
 
-#include "base/observer_list.h"
 #include "base/threading/non_thread_safe.h"
 #include "net/base/address_family.h"
 #include "net/base/host_cache.h"
@@ -47,8 +46,6 @@ class NET_EXPORT AsyncHostResolver
                                AddressList* addresses,
                                const BoundNetLog& source_net_log) OVERRIDE;
   virtual void CancelRequest(RequestHandle req_handle) OVERRIDE;
-  virtual void AddObserver(HostResolver::Observer* observer) OVERRIDE;
-  virtual void RemoveObserver(HostResolver::Observer* observer) OVERRIDE;
   virtual void SetDefaultAddressFamily(AddressFamily address_family) OVERRIDE;
   virtual AddressFamily GetDefaultAddressFamily() const OVERRIDE;
   virtual HostCache* GetHostCache() OVERRIDE;
@@ -145,10 +142,7 @@ class NET_EXPORT AsyncHostResolver
   // testing, outside of unit tests, its value is always NULL.
   ClientSocketFactory* factory_;
 
-  // The observers to notify when a request starts/ends.
-  ObserverList<HostResolver::Observer> observers_;
-
-  // Monotonically increasing ID number to assign to the next request.
+    // Monotonically increasing ID number to assign to the next request.
   // Observers are the only consumers of this ID number.
   int next_request_id_;