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

content/shell/browser/shell.h

Index: content/shell/browser/shell.h
diff --git a/content/shell/browser/shell.h b/content/shell/browser/shell.h
index f6dfe1be0e6104bd0e0b6bbc54ad7cd1f394670b..d62dfb5b2c2d919ad02e23518bf4636afa4f01d0 100644
--- a/content/shell/browser/shell.h
+++ b/content/shell/browser/shell.h
@@ -100,7 +100,7 @@ class Shell : public WebContentsDelegate,
 #if defined(OS_MACOSX)
   // Public to be called by an ObjC bridge object.
   void ActionPerformed(int control);
-  void URLEntered(std::string url_string);
+  void URLEntered(const std::string& url_string);
 #elif defined(OS_ANDROID)
   // Registers the Android Java to native methods.
   static bool Register(JNIEnv* env);