Remove unused HostResolver::Observer.

net/base/single_request_host_resolver_unittest.cc

Index: net/base/single_request_host_resolver_unittest.cc
diff --git a/net/base/single_request_host_resolver_unittest.cc b/net/base/single_request_host_resolver_unittest.cc
index ddf8c70e06ca091fec05720a9a48c3993464d3ed..7106af5f0560029b6fad7f271efb498fe1cf0def 100644
--- a/net/base/single_request_host_resolver_unittest.cc
+++ b/net/base/single_request_host_resolver_unittest.cc
@@ -57,14 +57,6 @@ class HangingHostResolver : public HostResolver {
     outstanding_request_ = NULL;
   }
 
-  virtual void AddObserver(Observer* observer) OVERRIDE {
-    FAIL();
-  }
-
-  virtual void RemoveObserver(Observer* observer) OVERRIDE {
-    FAIL();
-  }
-
  private:
   RequestHandle outstanding_request_;