First cut at "content handling" support in Mojo.

mojo/service_manager/background_shell_service_loader.cc

Index: mojo/service_manager/background_shell_service_loader.cc
diff --git a/mojo/service_manager/background_shell_service_loader.cc b/mojo/service_manager/background_shell_service_loader.cc
index 6d7a2582108628a29c9a728b9be3886ffe978b46..eab472769c22e71187b07e0793fa3dbe625a304f 100644
--- a/mojo/service_manager/background_shell_service_loader.cc
+++ b/mojo/service_manager/background_shell_service_loader.cc
@@ -18,7 +18,9 @@ class BackgroundShellServiceLoader::BackgroundLoader {
   void LoadService(ServiceManager* manager,
                    const GURL& url,
                    ScopedMessagePipeHandle shell_handle) {
-    loader_->LoadService(manager, url, shell_handle.Pass());
+    scoped_refptr<LoadServiceCallbacks> callbacks(
+        new ServiceLoader::SimpleLoadServiceCallbacks(shell_handle.Pass()));
+    loader_->LoadService(manager, url, callbacks);
   }
 
   void OnServiceError(ServiceManager* manager, const GURL& url) {
@@ -54,7 +56,11 @@ BackgroundShellServiceLoader::~BackgroundShellServiceLoader() {
 void BackgroundShellServiceLoader::LoadService(
     ServiceManager* manager,
     const GURL& url,
-    ScopedMessagePipeHandle shell_handle) {
+    scoped_refptr<LoadServiceCallbacks> callbacks) {
+  ScopedMessagePipeHandle shell_handle = callbacks->RegisterApplication();
+  if (!shell_handle.is_valid())
+    return;
+
   if (!thread_) {
     // TODO(tim): It'd be nice if we could just have each LoadService call
     // result in a new thread like DynamicService{Loader, Runner}. But some