Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/RasterWindowContext.h

Index: tools/viewer/sk_app/RasterWindowContext.h
diff --git a/tools/viewer/sk_app/RasterWindowContext.h b/tools/viewer/sk_app/RasterWindowContext.h
index f116b8eb1518973bbe3d34e3aa5f5d9247754b29..0393e9a0ba2ac10811e04e06f37ce3d5fd7773a6 100644
--- a/tools/viewer/sk_app/RasterWindowContext.h
+++ b/tools/viewer/sk_app/RasterWindowContext.h
@@ -14,9 +14,6 @@ namespace sk_app {
 
 class RasterWindowContext : public WindowContext {
 public:
-    // This is defined in the platform .cpp file
-    static RasterWindowContext* Create(void* platformData, const DisplayParams& params);
-
     // Explicitly convert nullptr to GrBackendContext is needed for compiling
     GrBackendContext getBackendContext() override { return (GrBackendContext) nullptr; }