Don't initialize the ZIP asset bundle as a directory. (#2932)

Regression introduced in https://github.com/flutter/engine/commit/cee7fef7

sky/shell/ui/engine.cc

@@ -90,7 +90,7 @@ void Engine::RunFromSource(const std::string& main,
   if (packages_path.empty())
     packages_path = FindPackagesPath(main);
   if (!bundle.empty())
-    ConfigureDirectoryAssetBundle(bundle);
+    ConfigureZipAssetBundle(bundle);
   ConfigureRuntime(main);
   runtime_->dart_controller()->RunFromSource(main, packages_path);
 }

sky/shell/ui/engine.h

@@ -92,8 +92,6 @@ class Engine : public UIDelegate,
   void RunFromSnapshotStream(const std::string& script_uri,
                              mojo::ScopedDataPipeConsumerHandle snapshot);
 
-  void SetupAssetBundle(const mojo::String& bundle_path);
-
   void StopAnimator();
   void StartAnimatorIfPossible();