Enable viewer in non-Vulkan builds

tools/viewer/sk_app/Window.cpp

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #include "Window.h"
 
 #include "SkSurface.h"
 #include "SkCanvas.h"
-#include "VulkanWindowContext.h"
+#include "WindowContext.h"
 
 namespace sk_app {
 
 static bool default_char_func(SkUnichar c, uint32_t modifiers, void* userData) {
     return false;
 }
 
 static bool default_key_func(Window::Key key, Window::InputState state, uint32_t modifiers, 
                              void* userData) {
     return false;
@@ skipping 88 matching lines @@
 
 void Window::markInvalProcessed() {
     fIsContentInvalidated = false;
 }
 
 sk_sp<SkSurface> Window::getOffscreenSurface(bool forceSRGB) {
     return fWindowContext->createOffscreenSurface(forceSRGB);
 }
 
 }   // namespace sk_app