Remove unused HostResolver::Observer.

net/base/mapped_host_resolver.cc

Index: net/base/mapped_host_resolver.cc
diff --git a/net/base/mapped_host_resolver.cc b/net/base/mapped_host_resolver.cc
index 5b8542f28d7bd51c8647c539b96c81a12d5a0e49..cb036f4f2f1ea530524e033327d2446be2a842b7 100644
--- a/net/base/mapped_host_resolver.cc
+++ b/net/base/mapped_host_resolver.cc
@@ -45,14 +45,6 @@ void MappedHostResolver::CancelRequest(RequestHandle req) {
   impl_->CancelRequest(req);
 }
 
-void MappedHostResolver::AddObserver(Observer* observer) {
-  impl_->AddObserver(observer);
-}
-
-void MappedHostResolver::RemoveObserver(Observer* observer) {
-  impl_->RemoveObserver(observer);
-}
-
 void MappedHostResolver::ProbeIPv6Support() {
   impl_->ProbeIPv6Support();
 }