PPAPI: Make blocking completion callbacks work.

ppapi/shared_impl/private/ppb_host_resolver_shared.cc

Index: ppapi/shared_impl/private/ppb_host_resolver_shared.cc
diff --git a/ppapi/shared_impl/private/ppb_host_resolver_shared.cc b/ppapi/shared_impl/private/ppb_host_resolver_shared.cc
index 581055d43105126a76c713a8a5595f42dc2e4546..fb44ad256a4555e5ee080e33cbfb8f0c05c07f19 100644
--- a/ppapi/shared_impl/private/ppb_host_resolver_shared.cc
+++ b/ppapi/shared_impl/private/ppb_host_resolver_shared.cc
@@ -53,15 +53,13 @@ int32_t PPB_HostResolver_Shared::Resolve(
     const char* host,
     uint16_t port,
     const PP_HostResolver_Private_Hint* hint,
-    PP_CompletionCallback callback) {
+    scoped_refptr<TrackedCallback> callback) {
   if (!host)
     return PP_ERROR_BADARGUMENT;
-  if (!callback.func)
-    return PP_ERROR_BLOCKS_MAIN_THREAD;
   if (ResolveInProgress())
     return PP_ERROR_INPROGRESS;
 
-  resolve_callback_ = new TrackedCallback(this, callback);
+  resolve_callback_ = callback;
 
   HostPortPair host_port;
   host_port.host = host;