Add base class for GLTestContext and add new subclass VkTestContext.

include/gpu/vk/GrVkDefines.h

Index: include/gpu/vk/GrVkDefines.h
diff --git a/include/gpu/vk/GrVkDefines.h b/include/gpu/vk/GrVkDefines.h
index 7ff81c84f912a78086f4aa6681af00257d16f51e..034d6d1a420ad7997ad099c8df436ce742728b5f 100644
--- a/include/gpu/vk/GrVkDefines.h
+++ b/include/gpu/vk/GrVkDefines.h
@@ -10,13 +10,22 @@
 #define GrVkDefines_DEFINED
 
 #if defined(SK_BUILD_FOR_WIN) || defined(SK_BUILD_FOR_WIN32)
-#define VK_USE_PLATFORM_WIN32_KHR
+#   define VK_USE_PLATFORM_WIN32_KHR
 #elif defined(SK_BUILD_FOR_ANDROID)
-#define VK_USE_PLATFORM_ANDROID_KHR
+#   define VK_USE_PLATFORM_ANDROID_KHR
 #elif defined(SK_BUILD_FOR_UNIX)
-#define VK_USE_PLATFORM_XLIB_KHR
+#   define VK_USE_PLATFORM_XLIB_KHR
+#endif
+
+#ifdef Bool
+#   pragma error "Bool is unexpectedly defined."
 #endif
 
 #include <vulkan/vulkan.h>
 
+// Xlib.h may define this macro (Grrr)
+#ifdef Bool
+#   undef Bool
+#endif
+
 #endif