Allow multiple logging::LogMessage{Handler,Listener}s

base/logging.h

Index: base/logging.h
diff --git a/base/logging.h b/base/logging.h
index fd148aa2694700e910e79810635e60fb466e2b56..86af81797901e7e749fc454ea1f3adbf91a5a9f1 100644
--- a/base/logging.h
+++ b/base/logging.h
@@ -282,11 +282,31 @@ BASE_EXPORT void SetLogAssertHandler(LogAssertHandlerFunction handler);
 // Sets the Log Message Handler that gets passed every log message before
 // it's sent to other log destinations (if any).
 // Returns true to signal that it handled the message and the message
-// should not be sent to other log destinations.
+// should not be sent to other log destinations, including other handlers
+// below the current one in the stack.
 typedef bool (*LogMessageHandlerFunction)(int severity,
     const char* file, int line, size_t message_start, const std::string& str);
-BASE_EXPORT void SetLogMessageHandler(LogMessageHandlerFunction handler);
-BASE_EXPORT LogMessageHandlerFunction GetLogMessageHandler();
+BASE_EXPORT void PushLogMessageHandler(LogMessageHandlerFunction handler);
+// When popping, GetTopLogMessageHandler() is supposed to be |expected|.
+BASE_EXPORT void PopLogMessageHandler(LogMessageHandlerFunction expected);
+BASE_EXPORT LogMessageHandlerFunction GetTopLogMessageHandler();
+
+// Sets the Log Message Listener that gets passed every log message before
+// it's sent to LogMessageHandlerFunctions. If hijacking message is not
+// needed, use this one instead of LogMessageHandlerFunction.
+typedef void (*LogMessageListenerFunction)(int severity,
+    const char* file, int line, size_t message_start, const std::string& str);
+
+class BASE_EXPORT ScopedLogMessageListener {
+ public:
+  explicit ScopedLogMessageListener(LogMessageListenerFunction listener);
+  ~ScopedLogMessageListener();
+ private:
+  LogMessageListenerFunction listener_;
+};
+
+BASE_EXPORT void AddLogMessageListener(LogMessageListenerFunction listener);
+BASE_EXPORT void RemoveLogMessageListener(LogMessageListenerFunction listener);
 
 typedef int LogSeverity;
 const LogSeverity LOG_VERBOSE = -1;  // This is level 1 verbosity