Remove BindStateHolder and have Bind() return a Callback<> object directly.

base/debug/trace_event.h

 // Copyright (c) 2011 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.
 
 // Trace events are for tracking application performance and resource usage.
 // Macros are provided to track:
 //    Begin and end of function calls
 //    Counters
 //
 // Events are issued against categories. Whereas LOG's
@@ skipping 733 matching lines @@
     std::string json_output;
   };
 
   TraceResultBuffer();
   ~TraceResultBuffer();
 
   // Set callback. The callback will be called during Start with the initial
   // JSON output and during AddFragment and Finish with following JSON output
   // chunks. The callback target must live past the last calls to
   // TraceResultBuffer::Start/AddFragment/Finish.
-  void SetOutputCallback(OutputCallback json_chunk_callback);
+  void SetOutputCallback(const OutputCallback& json_chunk_callback);
 
   // Start JSON output. This resets all internal state, so you can reuse
   // the TraceResultBuffer by calling Start.
   void Start();
 
   // Call AddFragment 0 or more times to add trace fragments from TraceLog.
   void AddFragment(const std::string& trace_fragment);
 
   // When all fragments have been added, call Finish to complete the JSON
   // formatted output.
@@ skipping 46 matching lines @@
   bool IsEnabled() { return enabled_; }
 
   float GetBufferPercentFull() const;
 
   // When enough events are collected, they are handed (in bulk) to
   // the output callback. If no callback is set, the output will be
   // silently dropped. The callback must be thread safe. The string format is
   // undefined. Use TraceResultBuffer to convert one or more trace strings to
   // JSON.
   typedef RefCountedData<std::string> RefCountedString;
-  typedef base::Callback<void(scoped_refptr<RefCountedString>)> OutputCallback;
+  typedef base::Callback<void(const scoped_refptr<RefCountedString>&)>
+      OutputCallback;
   void SetOutputCallback(const OutputCallback& cb);
 
   // The trace buffer does not flush dynamically, so when it fills up,
   // subsequent trace events will be dropped. This callback is generated when
   // the trace buffer is full. The callback must be thread safe.
   typedef base::Callback<void(void)> BufferFullCallback;
   void SetBufferFullCallback(const BufferFullCallback& cb);
 
   // Flushes all logged data to the callback.
   void Flush();
@@ skipping 155 matching lines @@
   Data* p_data_;
   Data data_;
 };
 
 }  // namespace internal
 
 }  // namespace debug
 }  // namespace base
 
 #endif  // BASE_DEBUG_TRACE_EVENT_H_