Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/win/WindowContextFactory_win.h

Index: tools/viewer/sk_app/win/WindowContextFactory_win.h
diff --git a/tools/viewer/sk_app/win/WindowContextFactory_win.h b/tools/viewer/sk_app/win/WindowContextFactory_win.h
new file mode 100644
index 0000000000000000000000000000000000000000..7aa4538c4b313d244759808f85977b226a9d54e9
--- /dev/null
+++ b/tools/viewer/sk_app/win/WindowContextFactory_win.h
@@ -0,0 +1,29 @@
+
+/*
+ * 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_win_DEFINED
+#define WindowContextFactory_win_DEFINED
+
+#include <Windows.h>
+
+namespace sk_app {
+
+class WindowContext;
+struct DisplayParams;
+
+namespace window_context_factory {
+
+WindowContext* NewVulkanForWin(HWND, const DisplayParams&);
+
+WindowContext* NewGLForWin(HWND, const DisplayParams&);
+
+}  // namespace window_context_factory
+
+}  // namespace sk_app
+
+#endif