Coverity: Fix several pass-by-values.

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

 /*
  * 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.
  */
 
 #define NACL_LOG_MODULE_NAME "Plugin::ServiceRuntime"
 
 #include "native_client/src/trusted/plugin/service_runtime.h"
 
@@ skipping 663 matching lines @@
     PLUGIN_PRINTF(("ServiceRuntime::SetupAppChannel (conect_desc=%p)\n",
                    static_cast<void*>(connect_desc)));
     SrpcClient* srpc_client = SrpcClient::New(connect_desc);
     PLUGIN_PRINTF(("ServiceRuntime::SetupAppChannel (srpc_client=%p)\n",
                    static_cast<void*>(srpc_client)));
     delete connect_desc;
     return srpc_client;
   }
 }
 
-bool ServiceRuntime::Log(int severity, nacl::string msg) {
+bool ServiceRuntime::Log(int severity, const nacl::string& msg) {
   NaClSrpcResultCodes rpc_result =
       NaClSrpcInvokeBySignature(&command_channel_,
                                 "log:is:",
                                 severity,
                                 strdup(msg.c_str()));
   return (NACL_SRPC_RESULT_OK == rpc_result);
 }
 
 void ServiceRuntime::Shutdown() {
   rev_interface_->ShutDown();
@@ skipping 49 matching lines @@
 
 nacl::string ServiceRuntime::GetCrashLogOutput() {
   if (NULL != subprocess_.get()) {
     return subprocess_->GetCrashLogOutput();
   } else {
     return "";
   }
 }
 
 }  // namespace plugin