drive: Change EventLogger to take printf format

chrome/browser/chromeos/drive/event_logger.h

 // Copyright (c) 2012 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 CHROME_BROWSER_CHROMEOS_DRIVE_EVENT_LOGGER_H_
 #define CHROME_BROWSER_CHROMEOS_DRIVE_EVENT_LOGGER_H_
 
+#include <stdarg.h>   // va_list
 #include <string>
 #include <deque>
 
 #include "base/basictypes.h"
+#include "base/compiler_specific.h"
 #include "base/time.h"
 
 namespace drive {
 
 // EventLogger is used to expose text messages to chrome:drive-internals,
 // for diagnosing the behavior of the Drive client.
 class EventLogger {
  public:
   // Represents a single event log.
   struct Event {
     Event(int id, const std::string& what);
     int id;  // Monotonically increasing ID starting from 0.
     base::Time when;  // When the event occurred.
     std::string what;  // What happened.
   };
 
   // Creates an event logger that keeps the latest |history_size| events.
   explicit EventLogger(size_t history_size);
   ~EventLogger();
 
-  // Logs a message.
-  void Log(const std::string& what);
+  // Logs a message using printf format.
+  // Note that PRINTF_FORMAT should be (2, 3) instead of (1, 2) as this is a
+  // C++ member function.
+  void Log(const char* format, ...) PRINTF_FORMAT(2, 3);
 
   // Gets the list of latest events (the oldest event comes first).
   const std::deque<Event>& history() const { return history_; }
 
  private:
   std::deque<Event> history_;
   const size_t history_size_;
   int next_event_id_;
 
   DISALLOW_COPY_AND_ASSIGN(EventLogger);
 };
 
 }  // namespace drive
 
 #endif  // CHROME_BROWSER_CHROMEOS_DRIVE_EVENT_LOGGER_H_