predictors: Downgrade net priority of prefetch requests.

chrome/browser/predictors/resource_prefetcher.cc

Index: chrome/browser/predictors/resource_prefetcher.cc
diff --git a/chrome/browser/predictors/resource_prefetcher.cc b/chrome/browser/predictors/resource_prefetcher.cc
index cc1d46616c556a1b415b8653e90a315283dd5634..69607a6e2f488d3920a8eae691f348b7d3f0259b 100644
--- a/chrome/browser/predictors/resource_prefetcher.cc
+++ b/chrome/browser/predictors/resource_prefetcher.cc
@@ -140,7 +140,7 @@ void ResourcePrefetcher::TryToLaunchPrefetchRequests() {
 
 void ResourcePrefetcher::SendRequest(const GURL& url) {
   std::unique_ptr<net::URLRequest> url_request =
-      delegate_->GetURLRequestContext()->CreateRequest(url, net::LOW, this);
+      delegate_->GetURLRequestContext()->CreateRequest(url, net::IDLE, this);
   host_inflight_counts_[url.host()] += 1;
 
   url_request->set_method("GET");