Cleanup: Pass std::string as const reference from native_client_sdk/

Passing std::string by reference can prevent extra copying of object.

BUG=367418
TEST=
R=binji@chromium.org,noelallen@chromium.org

Review URL: https://codereview.chromium.org/1355993002

Cr-Commit-Position: refs/heads/master@{#349917}

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);