Introduce NavigationSimulator to use in unit tests

content/browser/frame_host/navigation_handle_impl.cc

Index: content/browser/frame_host/navigation_handle_impl.cc
diff --git a/content/browser/frame_host/navigation_handle_impl.cc b/content/browser/frame_host/navigation_handle_impl.cc
index 74e0b202c6f005ffc79bb32c909b01bb847c83f2..437552e219975c24a754b1b8994759e35abc3891 100644
--- a/content/browser/frame_host/navigation_handle_impl.cc
+++ b/content/browser/frame_host/navigation_handle_impl.cc
@@ -647,7 +647,9 @@ void NavigationHandleImpl::Transfer() {
   // 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();
 }