Coverity: Fix several pass-by-values.

ppapi/native_client/src/trusted/plugin/plugin.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.
 
 // The portable representation of an instance and root scriptable object.
 // The PPAPI version of the plugin instantiates a subclass of this class.
 
 #ifndef NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_PLUGIN_H_
 #define NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_PLUGIN_H_
@@ skipping 184 matching lines @@
   int argc() const { return argc_; }
   char** argn() const { return argn_; }
   char** argv() const { return argv_; }
 
   Plugin* plugin() const { return const_cast<Plugin*>(this); }
 
   // URL resolution support.
   // plugin_base_url is the URL used for resolving relative URLs used in
   // src="...".
   nacl::string plugin_base_url() const { return plugin_base_url_; }
-  void set_plugin_base_url(nacl::string url) { plugin_base_url_ = url; }
+  void set_plugin_base_url(const nacl::string& url) { plugin_base_url_ = url; }
   // manifest_base_url is the URL used for resolving relative URLs mentioned
   // in manifest files.  If the manifest is a data URI, this is an empty string.
   nacl::string manifest_base_url() const { return manifest_base_url_; }
-  void set_manifest_base_url(nacl::string url) { manifest_base_url_ = url; }
+  void set_manifest_base_url(const nacl::string& url) {
+    manifest_base_url_ = url;
+  }
 
   // The URL of the manifest file as set by the "src" attribute.
   // It is not the fully resolved URL if it was set as relative.
   const nacl::string& manifest_url() const { return manifest_url_; }
   void set_manifest_url(const nacl::string& manifest_url) {
     manifest_url_ = manifest_url;
   }
 
   // The state of readiness of the plugin.
   enum ReadyState {
@@ skipping 307 matching lines @@
 
   // Whether we are using IPC-based PPAPI proxy.
   bool using_ipc_proxy_;
 
   const PPB_NaCl_Private* nacl_interface_;
 };
 
 }  // namespace plugin
 
 #endif  // NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_PLUGIN_H_