FBTF: Remove unneeded headers from base/ (part 2)

ipc/ipc_sync_message_filter.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 IPC_IPC_SYNC_MESSAGE_FILTER_H_
 #define IPC_IPC_SYNC_MESSAGE_FILTER_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/lock.h"
 #include "base/ref_counted.h"
-#include "base/waitable_event.h"
 #include "ipc/ipc_channel_proxy.h"
 #include "ipc/ipc_sync_message.h"
 #include <set>
 
+namespace base {
+class WaitableEvent;
+}
+
 class MessageLoop;
 
 namespace IPC {
 
 class MessageReplyDeserializer;
 
 // This MessageFilter allows sending synchronous IPC messages from a thread
 // other than the listener thread associated with the SyncChannel.  It does not
 // support fancy features that SyncChannel does, such as handling recursion or
 // receiving messages while waiting for a response.  Note that this object can
@@ skipping 31 matching lines @@
   Lock lock_;
 
   base::WaitableEvent* shutdown_event_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncMessageFilter);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_SYNC_MESSAGE_FILTER_H_