Add support for using alternative backends (like DirectX) when creating a GrGpu.

src/gpu/GrGpuFactory.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrGpuFactory_DEFINED
+#define GrGpuFactory_DEFINED
+
+#include "GrTypes.h"
+
+class GrGpu;
+class GrContext;
+
+typedef GrGpu* (*CreateGpuProc)(GrBackendContext, GrContext*);
+
+class GrGpuFactoryRegistrar {
+public:
+    GrGpuFactoryRegistrar(int i, CreateGpuProc proc);
+};
+
+#endif