Introduce NavigationSimulator to use in unit tests

content/browser/frame_host/navigation_handle_impl.cc

 // Copyright 2015 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 "content/browser/frame_host/navigation_handle_impl.h"
 
 #include <iterator>
 
 #include "base/debug/dump_without_crashing.h"
 #include "base/logging.h"
@@ skipping 629 matching lines @@
   }
 }
 
 void NavigationHandleImpl::Transfer() {
   DCHECK(!IsBrowserSideNavigationEnabled());
   // This is an actual transfer. Inform the NavigationResourceThrottle. This
   // will allow to mark the URLRequest as transferring. When it is marked as
   // transferring, the URLRequest can no longer be cancelled by its original
   // RenderFrame. Instead it will persist until being picked up by the transfer
   // RenderFrame, even if the original RenderFrame is destroyed.
-  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE, transfer_callback_);
+  // Note: |transfer_callback_| can be null in unit tests.
+  if (!transfer_callback_.is_null())
+    BrowserThread::PostTask(BrowserThread::IO, FROM_HERE, transfer_callback_);
   transfer_callback_.Reset();
 }
 
 NavigationThrottle::ThrottleCheckResult
 NavigationHandleImpl::CheckWillStartRequest() {
   DCHECK(state_ == WILL_SEND_REQUEST || state_ == DEFERRING_START);
   DCHECK(state_ != WILL_SEND_REQUEST || next_index_ == 0);
   DCHECK(state_ != DEFERRING_START || next_index_ != 0);
   for (size_t i = next_index_; i < throttles_.size(); ++i) {
     NavigationThrottle::ThrottleCheckResult result =
@@ skipping 222 matching lines @@
       content::AncestorThrottle::MaybeCreateThrottleFor(this);
   if (ancestor_throttle)
     throttles_.push_back(std::move(ancestor_throttle));
 
   throttles_.insert(throttles_.begin(),
                     std::make_move_iterator(throttles_to_register.begin()),
                     std::make_move_iterator(throttles_to_register.end()));
 }
 
 }  // namespace content