[ios] Change API to inform WebStateList of opener-opened relationship.

ios/shared/chrome/browser/tabs/web_state_list_fast_enumeration_helper_unittest.mm

 // Copyright 2017 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.
 
 #import "ios/shared/chrome/browser/tabs/web_state_list_fast_enumeration_helper.h"
 
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #import "ios/shared/chrome/browser/tabs/fake_web_state_list_delegate.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list.h"
@@ skipping 51 matching lines @@
     }
     return [array copy];
   }
 
   void InsertTestWebStateForURLs(const char* const* urls, size_t count) {
     for (size_t index = 0; index < count; ++index) {
       auto test_web_state = base::MakeUnique<web::TestWebState>();
       test_web_state->SetCurrentURL(GURL(urls[index]));
 
       web_state_list_.InsertWebState(web_state_list_.count(),
-                                     test_web_state.release(), nullptr);
+                                     test_web_state.release());
     }
   }
 
  private:
   FakeWebStateListDelegate web_state_list_delegate_;
   WebStateList web_state_list_;
 
   DISALLOW_COPY_AND_ASSIGN(WebStateListFastEnumerationHelperTest);
 };
 
@@ skipping 10 matching lines @@
     EXPECT_NSEQ([NSURL URLWithString:@(kURLs[index])], array[index]);
   }
 }
 
 TEST_F(WebStateListFastEnumerationHelperTest, NilWrapper) {
   const char* const urls[] = {""};
   InsertTestWebStateForURLs(urls, arraysize(urls));
   NSArray* array = ArrayFromWebStateList();
   ASSERT_EQ(0u, [array count]);
 }