base: Cleanup class/struct forward declarations

base/trace_event/trace_log.h

 // Copyright 2015 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 BASE_TRACE_EVENT_TRACE_LOG_H_
 #define BASE_TRACE_EVENT_TRACE_LOG_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/atomicops.h"
 #include "base/containers/hash_tables.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/scoped_vector.h"
 #include "base/trace_event/memory_dump_provider.h"
 #include "base/trace_event/trace_config.h"
 #include "base/trace_event/trace_event_impl.h"
 #include "build/build_config.h"
 
 namespace base {
 
 template <typename Type>
 struct DefaultSingletonTraits;
+class MessageLoop;
 class RefCountedString;
 
 namespace trace_event {
 
 class TraceBuffer;
 class TraceBufferChunk;
 class TraceEvent;
 class TraceEventMemoryOverhead;
 
 struct BASE_EXPORT TraceLogStatus {
@@ skipping 481 matching lines @@
   subtle::AtomicWord generation_;
   bool use_worker_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceLog);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_LOG_H_