Rename VulkanViewer to Viewer, take 2.

tools/viewer/Application.h

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #ifndef Application_DEFINED
 #define Application_DEFINED
 
+namespace sk_app {
+
 class Application {
 public:
     static Application* Create(int argc, char** argv, void* platformData);
 
     virtual ~Application() {}
 
     virtual void onIdle(double ms) = 0;
 };
 
+}   // namespace sk_app
+
 #endif