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 f06535dada056725121d0a31c0c745bca9c8f0f0..2be7e07131491ab16caafd8b7b7a982ab4f9ad68 100644
--- a/base/logging.h
+++ b/base/logging.h
@@ -285,8 +285,9 @@ BASE_EXPORT void SetLogAssertHandler(LogAssertHandlerFunction handler);
 // should not be sent to other log destinations.
 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);
+BASE_EXPORT void PopLogMessageHandler();
+BASE_EXPORT LogMessageHandlerFunction GetTopLogMessageHandler();
 
 typedef int LogSeverity;
 const LogSeverity LOG_VERBOSE = -1;  // This is level 1 verbosity