Setting search prefetch priority to MEDIUM/HIGHEST

When the prefetch request starts, we will use MEDIUM priority, when it
is being served, we will convert it to HIGHEST prioirty (derived from
navigation resource request).

Bug: 11567701
Change-Id: I388336e972675ea72504135c0022c9b56aa3a99c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580447
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834978}

chrome/browser/prefetch/search_prefetch/base_search_prefetch_request.cc

@@ -126,6 +126,10 @@ bool BaseSearchPrefetchRequest::StartPrefetchRequest(Profile* profile) {
   url::Origin prefetch_origin = url::Origin::Create(prefetch_url_);
 
   auto resource_request = std::make_unique<network::ResourceRequest>();
+  // This prefetch is not as high priority as navigation, but due to its
+  // navigation speeding and relatively high likelihood of being served to a
+  // navigation, the request is relatively high priority.
+  resource_request->priority = net::MEDIUM;
   resource_request->load_flags |= net::LOAD_PREFETCH;
   resource_request->url = prefetch_url_;
   // Search prefetch URL Loaders should check |report_raw_headers| on the

chrome/browser/prefetch/search_prefetch/streaming_search_prefetch_url_loader.cc

@@ -64,6 +64,9 @@ void StreamingSearchPrefetchURLLoader::SetUpForwardingClient(
   DCHECK(!streaming_prefetch_request_);
   // Bind to the content/ navigation code.
   DCHECK(!receiver_.is_bound());
+
+  network_url_loader_->SetPriority(resource_request.priority, -1);
+
   receiver_.Bind(std::move(receiver));
   receiver_.set_disconnect_handler(
       base::BindOnce(&StreamingSearchPrefetchURLLoader::OnMojoDisconnect,