Rename ViewportObserver to SkyEngine

sky/services/engine/sky_engine.mojom

 // Copyright 2015 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 sky;
 
-import "sky/services/viewport/input_event.mojom";
+import "sky/services/engine/input_event.mojom";
 
 // TODO(eseidel): Rename this interface.
-interface ViewportObserver {
+interface SkyEngine {
   OnViewportMetricsChanged(int32 width, int32 height, float device_pixel_ratio);
   OnInputEvent(InputEvent event);
 
   RunFromNetwork(string url);
   RunFromFile(string main, string package_root);
   RunFromSnapshot(string path);
 };