Add base class for GLTestContext and add new subclass VkTestContext.

tools/gpu/vk/VkTestContext.h

+/*
+ * 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 VkTestContext_DEFINED
+#define VkTestContext_DEFINED
+
+#include "TestContext.h"
+#include "vk/GrVkBackendContext.h"
+
+#ifdef SK_VULKAN
+
+namespace sk_gpu_test {
+class VkTestContext : public TestContext {
+public:
+    bool isValid() const override { return NULL != this->vk(); }
+
+    const GrVkInterface* vk() const { return fVk->fInterface; };
+
+protected:
+    VkTestContext(sk_sp<const GrVkBackendContext> vk) : fVk(std::move(vk)) {};
+
+    sk_sp<const GrVkBackendContext> fVk;
+
+private:
+    typedef TestContext INHERITED;
+};
+
+/**
+ * Creates Vk context object bound to the native Vk library.
+ */
+VkTestContext* CreatePlatformVkTestContext();
+
+}  // namespace sk_gpu_test
+
+#endif
+
+#endif