Constify SchemeHostPort and Origin accessors.

url/scheme_host_port.h

Index: url/scheme_host_port.h
diff --git a/url/scheme_host_port.h b/url/scheme_host_port.h
index 227c1f17ae157dabc39640589fb15f2814930ff3..2cc9e070db868cb5631a6c8ffc03c2a09af8fbfd 100644
--- a/url/scheme_host_port.h
+++ b/url/scheme_host_port.h
@@ -96,8 +96,8 @@ class URL_EXPORT SchemeHostPort {
   // Returns the host component, in URL form. That is all IDN domain names will
   // be expressed as A-Labels ('☃.net' will be returned as 'xn--n3h.net'), and
   // and all IPv6 addresses will be enclosed in brackets ("[2001:db8::1]").
-  std::string host() const { return host_; }
-  std::string scheme() const { return scheme_; }
+  const std::string& host() const { return host_; }
+  const std::string& scheme() const { return scheme_; }
   uint16 port() const { return port_; }
   bool IsInvalid() const;