Convert Pass()→std::move() for iOS build.

ios/chrome/browser/signin/signin_client_impl.cc

Index: ios/chrome/browser/signin/signin_client_impl.cc
diff --git a/ios/chrome/browser/signin/signin_client_impl.cc b/ios/chrome/browser/signin/signin_client_impl.cc
index 12956680e0655e6cbfec5231774cbca71da319d5..5a18bba059b743bfdd7adfd48c95c7a5e5211846 100644
--- a/ios/chrome/browser/signin/signin_client_impl.cc
+++ b/ios/chrome/browser/signin/signin_client_impl.cc
@@ -6,6 +6,8 @@
 
 #include <stddef.h>
 
+#include <utility>
+
 #include "base/command_line.h"
 #include "base/guid.h"
 #include "base/logging.h"
@@ -142,7 +144,7 @@ SigninClientImpl::AddCookieChangedCallback(
   DCHECK(context_getter.get());
   scoped_ptr<SigninCookieChangedSubscription> subscription(
       new SigninCookieChangedSubscription(context_getter, url, name, callback));
-  return subscription.Pass();
+  return std::move(subscription);
 }
 
 void SigninClientImpl::OnSignedIn(const std::string& account_id,