Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/unix/WindowContextFactory_unix.h

Index: tools/viewer/sk_app/unix/WindowContextFactory_unix.h
diff --git a/tools/viewer/sk_app/unix/WindowContextFactory_unix.h b/tools/viewer/sk_app/unix/WindowContextFactory_unix.h
new file mode 100644
index 0000000000000000000000000000000000000000..fb1568902d5b8aefc376565b9715d6af4bfc0d95
--- /dev/null
+++ b/tools/viewer/sk_app/unix/WindowContextFactory_unix.h
@@ -0,0 +1,37 @@
+
+/*
+ * 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_unix_DEFINED
+#define WindowContextFactory_unix_DEFINED
+
+#include <X11/Xlib.h>
+#include <GL/glx.h>
+typedef Window XWindow;
+
+namespace sk_app {
+
+class WindowContext;
+struct DisplayParams;
+
+namespace window_context_factory {
+
+struct XlibWindowInfo {
+    Display*     fDisplay;
+    XWindow      fWindow;
+    XVisualInfo* fVisualInfo;
+};
+
+WindowContext* NewVulkanForXlib(const XlibWindowInfo&, const DisplayParams&);
+
+WindowContext* NewGLForXlib(const XlibWindowInfo&, const DisplayParams&);
+
+}  // namespace window_context_factory
+
+}  // namespace sk_app
+
+#endif