Declarative resource hints go through mojo IPC to //content

third_party/WebKit/public/platform/resource_hints.mojom

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module blink.mojom;
+
+import "url/mojo/url.mojom";
+
+// The ResourceHintsHandler sends preconnect/preresolve requests based
+// on link tags/headers. See the corresponding interface in
+// content/public/resource_hints.h.
+// TODO(csharrison): Extend this interface to include page scans and link
+// hovers.
+interface ResourceHintsHandler {
+  // Issues |num_connections| preconnects for the given URL. This ensures that
+  // at least |num_connections| are connected to the host in |url|. Note that
+  // if sockets are already connected, the net stack will only connect up to
+  // |num_connections|, so calling this multiple times is "safe". The
+  // |credentials_flag| corresponds to the fetch API and its notion of
+  // credentialed requests.
+  Preconnect(url.mojom.Url url, bool credentials_flag, uint8 num_connections);
+
+  // Resolves the given URL's host, so subsequent requests will hit the host
+  // cache.
+  Preresolve(url.mojom.Url url);
+};