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 a5459d616a3ec8087786bbe6be27ca47b1aa541f..464fa14b8e73787c24bef218066bd4c993f432a1 100644
--- a/content/browser/frame_host/navigation_handle_impl.cc
+++ b/content/browser/frame_host/navigation_handle_impl.cc
@@ -659,7 +659,9 @@ 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();
 }