Fixing a crashy race conidtion in search prefetch

Since there can be a time gap between when the url loader interceptor is
handed off to content/ and when the throttles allow the request to
proceed, there is a possibility of forwarding_client_ not being set up
while the streaming request pointer is null.

I suspect this is where the crash is coming from.

Bug: 1164190
Change-Id: I1e68fa7898737ba71b68ec681903b88ece88c8d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617858
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Auto-Submit: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841579}

chrome/browser/prefetch/search_prefetch/streaming_search_prefetch_url_loader.cc

@@ -270,8 +270,8 @@ void StreamingSearchPrefetchURLLoader::OnComplete(
     return;
   }
 
-  if (!forwarding_client_) {
-    DCHECK(streaming_prefetch_request_);
+  if (streaming_prefetch_request_) {
+    DCHECK(!forwarding_client_);
     if (status.error_code == net::OK) {
       streaming_prefetch_request_->MarkPrefetchAsComplete();
     } else {