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

net/spdy/spdy_protocol.h

Index: net/spdy/spdy_protocol.h
diff --git a/net/spdy/spdy_protocol.h b/net/spdy/spdy_protocol.h
index 2e4fde870997ba5f8cb1f2fc73e3f7f6347a4087..59985ede0b2a7c87aacb0209bf91cb346ede29da 100644
--- a/net/spdy/spdy_protocol.h
+++ b/net/spdy/spdy_protocol.h
@@ -969,7 +969,7 @@ class NET_EXPORT_PRIVATE SpdyAltSvcIR : public SpdyFrameWithStreamIdIR {
     return altsvc_vector_;
   }
 
-  void set_origin(const std::string origin) { origin_ = origin; }
+  void set_origin(const std::string& origin) { origin_ = origin; }
   void add_altsvc(const SpdyAltSvcWireFormat::AlternativeService& altsvc) {
     altsvc_vector_.push_back(altsvc);
   }