Coverity: Fix several pass-by-values.

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

Index: ppapi/native_client/src/trusted/plugin/plugin.h
diff --git a/ppapi/native_client/src/trusted/plugin/plugin.h b/ppapi/native_client/src/trusted/plugin/plugin.h
index ce6df11287d4db966732459b86a07a4cdc700a9d..71ff2901643596b73c594de1711b047dcc3f6f8e 100644
--- a/ppapi/native_client/src/trusted/plugin/plugin.h
+++ b/ppapi/native_client/src/trusted/plugin/plugin.h
@@ -202,11 +202,13 @@ class Plugin : public pp::InstancePrivate {
   // 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.