Make CanonicalCookie::Source() private.

ios/net/cookies/system_cookie_util_unittest.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ios/net/cookies/system_cookie_util.h"
 
 #import <Foundation/Foundation.h>
 
 #include "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
@@ skipping 56 matching lines @@
         NSHTTPCookieDomain : @"foo",
         NSHTTPCookieName : @"a",
         NSHTTPCookiePath : @"/",
         NSHTTPCookieValue : @"b",
         NSHTTPCookieExpires : system_expire_date,
         @"HttpOnly" : @YES,
       }]);
   ASSERT_TRUE(system_cookie);
   net::CanonicalCookie chrome_cookie =
       CanonicalCookieFromSystemCookie(system_cookie, creation_time);
-  EXPECT_TRUE(chrome_cookie.Source().is_empty());
   EXPECT_EQ("a", chrome_cookie.Name());
   EXPECT_EQ("b", chrome_cookie.Value());
   EXPECT_EQ("foo", chrome_cookie.Domain());
   EXPECT_EQ("/", chrome_cookie.Path());
   EXPECT_EQ(creation_time, chrome_cookie.CreationDate());
   EXPECT_TRUE(chrome_cookie.LastAccessDate().is_null());
   EXPECT_TRUE(chrome_cookie.IsPersistent());
   // Allow 1 second difference as iOS rounds expiry time to the nearest second.
   EXPECT_LE(expire_date - base::TimeDelta::FromSeconds(1),
             chrome_cookie.ExpiryDate());
@@ skipping 37 matching lines @@
       false,         // secure
       false,         // httponly
       net::CookieSameSite::DEFAULT_MODE, net::COOKIE_PRIORITY_DEFAULT);
   // Convert it to system cookie.
   base::scoped_nsobject<NSHTTPCookie> system_cookie(
       [SystemCookieFromCanonicalCookie(bad_canonical_cookie) retain]);
   EXPECT_TRUE(system_cookie == nil);
 }
 
 }  // namespace net