Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/mac/WindowContextFactory_mac.h

Index: tools/viewer/sk_app/mac/WindowContextFactory_mac.h
diff --git a/tools/viewer/sk_app/mac/WindowContextFactory_mac.h b/tools/viewer/sk_app/mac/WindowContextFactory_mac.h
new file mode 100644
index 0000000000000000000000000000000000000000..a86fc738352ae0e5ca9d863b67bc5f6b02204732
--- /dev/null
+++ b/tools/viewer/sk_app/mac/WindowContextFactory_mac.h
@@ -0,0 +1,38 @@
+
+/*
+ * 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_mac_DEFINED
+#define WindowContextFactory_mac_DEFINED
+
+namespace sk_app {
+
+class WindowContext;
+struct DisplayParams;
+
+namespace window_context_factory {
+
+struct MacWindowInfo {
+#if 0 // TODO: use Mac-specific objects
+    Display*     fDisplay;
+    XWindow      fWindow;
+    XVisualInfo* fVisualInfo;
+#endif
+};
+
+inline WindowContext* NewVulkanForMac(const MacWindowInfo&, const DisplayParams&) {
+    // No Vulkan support on Mac.
+    return nullptr;
+}
+
+WindowContext* NewGLForMac(const MacWindowInfo&, const DisplayParams&);
+
+}  // namespace window_context_factory
+
+}  // namespace sk_app
+
+#endif