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

chrome/browser/media/webrtc_browsertest_base.h

Index: chrome/browser/media/webrtc_browsertest_base.h
diff --git a/chrome/browser/media/webrtc_browsertest_base.h b/chrome/browser/media/webrtc_browsertest_base.h
index 493807bce6d232e4bf2fe1a280ed488e23fae5fa..4f043573d25e652ea938b5dcba74bca05bfb18e7 100644
--- a/chrome/browser/media/webrtc_browsertest_base.h
+++ b/chrome/browser/media/webrtc_browsertest_base.h
@@ -141,7 +141,8 @@ class WebRtcTestBase : public InProcessBrowserTest {
   std::string CreateLocalOffer(content::WebContents* from_tab) const;
   std::string CreateAnswer(std::string local_offer,
                            content::WebContents* to_tab) const;
-  void ReceiveAnswer(std::string answer, content::WebContents* from_tab) const;
+  void ReceiveAnswer(const std::string& answer,
+                     content::WebContents* from_tab) const;
   void GatherAndSendIceCandidates(content::WebContents* from_tab,
                                   content::WebContents* to_tab) const;