Enable viewer in non-Vulkan builds

tools/viewer/sk_app/Window.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 Window_DEFINED
 #define Window_DEFINED
 
@@ skipping 24 matching lines @@
     // Make sure that either onPaint or markInvalReceived is called when the client window consumes
     // the the inval event. They unset fIsContentInvalided which allow future onInval.
     void inval();
 
     virtual bool scaleContentToFit() const { return false; }
     virtual bool supportsContentRect() const { return false; }
     virtual SkRect getContentRect() { return SkRect::MakeEmpty(); }
 
     enum BackendType {
         kNativeGL_BackendType,
+#ifdef SK_VULKAN
         kVulkan_BackendType,
+#endif
         kRaster_BackendType,
 
         kLast_BackendType = kRaster_BackendType
     };
     enum {
         kBackendTypeCount = kLast_BackendType + 1
     };
 
     virtual bool attach(BackendType attachType,  const DisplayParams& params) = 0;
     void detach();
@@ skipping 134 matching lines @@
     virtual void onInval() = 0;
 
     // Uncheck fIsContentInvalided to allow future inval/onInval.
     void markInvalProcessed();
 
     bool fIsContentInvalidated = false;  // use this to avoid duplicate invalidate events
 };
 
 }   // namespace sk_app
 #endif