Cleanup: Pass std::string as const reference if possible

native_client_sdk/src/examples/api/file_io/file_io.cc

Index: native_client_sdk/src/examples/api/file_io/file_io.cc
diff --git a/native_client_sdk/src/examples/api/file_io/file_io.cc b/native_client_sdk/src/examples/api/file_io/file_io.cc
index 2f3eaaeff1d38012e90857816aca5c62816c0dbc..01a06ff3071f2b535e05d5b3335bb217e3c7e7c0 100644
--- a/native_client_sdk/src/examples/api/file_io/file_io.cc
+++ b/native_client_sdk/src/examples/api/file_io/file_io.cc
@@ -100,7 +100,7 @@ class FileIoInstance : public pp::Instance {
     PostArrayMessage(command, StringVector());
   }
 
-  void PostArrayMessage(const char* command, std::string s) {
+  void PostArrayMessage(const char* command, const std::string& s) {
     StringVector sv;
     sv.push_back(s);
     PostArrayMessage(command, sv);