Remove obviously unneeded forward declarations from headers under src/{app,base,net}.

net/base/host_resolver.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_BASE_HOST_RESOLVER_H_
 #define NET_BASE_HOST_RESOLVER_H_
 #pragma once
 
 #include <string>
 
 #include "base/ref_counted.h"
 #include "googleurl/src/gurl.h"
 #include "net/base/address_family.h"
 #include "net/base/completion_callback.h"
 #include "net/base/request_priority.h"
 
-class MessageLoop;
-
 namespace net {
 
 class AddressList;
 class BoundNetLog;
-class HostCache;
 class HostResolverImpl;
 
 // This class represents the task of resolving hostnames (or IP address
 // literal) to an AddressList object.
 //
 // HostResolver can handle multiple requests at a time, so when cancelling a
 // request the RequestHandle that was returned by Resolve() needs to be
 // given.  A simpler alternative for consumers that only have 1 outstanding
 // request at a time is to create a SingleRequestHostResolver wrapper around
 // HostResolver (which will automatically cancel the single request when it
@@ skipping 211 matching lines @@
 // (Except if a unit-test has changed the global HostResolverProc using
 // ScopedHostResolverProc to intercept requests to the system).
 // |max_concurrent_resolves| is how many resolve requests will be allowed to
 // run in parallel. Pass HostResolver::kDefaultParallelism to choose a
 // default value.
 HostResolver* CreateSystemHostResolver(size_t max_concurrent_resolves);
 
 }  // namespace net
 
 #endif  // NET_BASE_HOST_RESOLVER_H_