Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/android/WindowContextFactory_android.h

Index: tools/viewer/sk_app/android/WindowContextFactory_android.h
diff --git a/tools/viewer/sk_app/android/WindowContextFactory_android.h b/tools/viewer/sk_app/android/WindowContextFactory_android.h
new file mode 100644
index 0000000000000000000000000000000000000000..00198da8d3b7437ebd51badaee58c50304d655e5
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+++ b/tools/viewer/sk_app/android/WindowContextFactory_android.h
@@ -0,0 +1,32 @@
+
+/*
+ * 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 WindowContextFactory_android_DEFINED
+#define WindowContextFactory_android_DEFINED
+
+#include <android/native_window_jni.h>
+
+
+namespace sk_app {
+
+class WindowContext;
+struct DisplayParams;
+
+namespace window_context_factory {
+
+WindowContext* NewVulkanForAndroid(ANativeWindow*, const DisplayParams&);
+
+WindowContext* NewGLForAndroid(ANativeWindow*, const DisplayParams&);
+
+WindowContext* NewRasterForAndroid(ANativeWindow*, const DisplayParams&);
+
+}  // namespace window_context_factory
+
+}  // namespace sk_app
+
+#endif