Cleanup return values in http_auth

This CL turns const by value return values into const by ref

BUG=393155

Review URL: https://codereview.chromium.org/1105093002

Cr-Commit-Position: refs/heads/master@{#327239}

net/http/http_auth_cache.h

@@ -36,9 +36,7 @@ class NET_EXPORT_PRIVATE HttpAuthCache {
     }
 
     // The case-sensitive realm string of the challenge.
-    const std::string realm() const {
-      return realm_;
-    }
+    const std::string& realm() const { return realm_; }
 
     // The authentication scheme of the challenge.
     HttpAuth::Scheme scheme() const {
@@ -46,9 +44,7 @@ class NET_EXPORT_PRIVATE HttpAuthCache {
     }
 
     // The authentication challenge.
-    const std::string auth_challenge() const {
-      return auth_challenge_;
-    }
+    const std::string& auth_challenge() const { return auth_challenge_; }
 
     // The login credentials.
     const AuthCredentials& credentials() const {

net/http/http_auth_handler.h

@@ -87,9 +87,7 @@ class NET_EXPORT_PRIVATE HttpAuthHandler {
   }
 
   // The challenge which was issued when creating the handler.
-  const std::string challenge() const {
-    return auth_challenge_;
-  }
+  const std::string& challenge() const { return auth_challenge_; }
 
   // Numeric rank based on the challenge's security level. Higher
   // numbers are better. Used by HttpAuth::ChooseBestChallenge().