Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/android/WindowContext_android.h

Index: tools/viewer/sk_app/android/WindowContext_android.h
diff --git a/tools/viewer/sk_app/android/WindowContext_android.h b/tools/viewer/sk_app/android/WindowContext_android.h
deleted file mode 100644
index f0529fe44eee6356a39283e318115a4e653e7adc..0000000000000000000000000000000000000000
--- a/tools/viewer/sk_app/android/WindowContext_android.h
+++ /dev/null
@@ -1,21 +0,0 @@
-/*
-* 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 WindowContext_android_DEFINED
-#define WindowContext_android_DEFINED
-
-#include <android/native_window_jni.h>
-
-namespace sk_app {
-
-struct ContextPlatformData_android {
-    ANativeWindow* fNativeWindow;
-};
-
-}   // namespace sk_app
-
-#endif // WindowContext_android_DEFINED