Add a Dns preresolve interface in //content

net/dns/single_request_host_resolver.h

Index: net/dns/single_request_host_resolver.h
diff --git a/net/dns/single_request_host_resolver.h b/net/dns/single_request_host_resolver.h
index f2843093fc8ec1ba761cdd67938184180774b8e4..06f9fe2c5877f496a87e945ec44a01cae748f606 100644
--- a/net/dns/single_request_host_resolver.h
+++ b/net/dns/single_request_host_resolver.h
@@ -31,6 +31,8 @@ class NET_EXPORT SingleRequestHostResolver {
 
   // Resolves the given hostname (or IP address literal), filling out the
   // |addresses| object upon success. See HostResolver::Resolve() for details.
+  // Note, |addresses| can be null if the caller does not care about the return
+  // resolution result, and only wishes to perform the resolution.
   int Resolve(const HostResolver::RequestInfo& info,
               RequestPriority priority,
               AddressList* addresses,
@@ -56,6 +58,8 @@ class NET_EXPORT SingleRequestHostResolver {
   // Completion callback for when request to |resolver_| completes.
   CompletionCallback callback_;
 
+  std::unique_ptr<AddressList> default_addresses_;
+
   DISALLOW_COPY_AND_ASSIGN(SingleRequestHostResolver);
 };