Coverity: Fix several pass-by-values.

ppapi/native_client/src/trusted/plugin/service_runtime.h

 /* -*- c++ -*- */
 /*
  * 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.
  */
 
 // A class containing information regarding a socket connection to a
 // service runtime instance.
 
@@ skipping 206 matching lines @@
   // to be started is passed through |nacl_file_desc|.  On success, returns
   // true.  On failure, returns false and |error_string| is set to something
   // describing the error.
   bool Start(nacl::DescWrapper* nacl_file_desc,
              ErrorInfo* error_info,
              const nacl::string& url);
 
   // Starts the application channel to the nexe.
   SrpcClient* SetupAppChannel();
 
-  bool Log(int severity, nacl::string msg);
+  bool Log(int severity, const nacl::string& msg);
   Plugin* plugin() const { return plugin_; }
   void Shutdown();
 
   // exit_status is -1 when invalid; when we set it, we will ensure
   // that it is non-negative (the portion of the exit status from the
   // nexe that is transferred is the low 8 bits of the argument to the
   // exit syscall).
   int exit_status();  // const, but grabs mutex etc.
   void set_exit_status(int exit_status);
 
@@ skipping 17 matching lines @@
 
   PluginReverseInterface* rev_interface_;
 
   NaClMutex mu_;
   int exit_status_;
 };
 
 }  // namespace plugin
 
 #endif  // NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_SERVICE_RUNTIME_H_