Add flag for window rectangles to GrRenderTarget

src/gpu/vk/GrVkRenderTarget.h

Index: src/gpu/vk/GrVkRenderTarget.h
diff --git a/src/gpu/vk/GrVkRenderTarget.h b/src/gpu/vk/GrVkRenderTarget.h
index 019f56743322c50d03193f1d013f03a1add40be2..88da283c9a65d1147b71661c86b9d3f18a9c2610 100644
--- a/src/gpu/vk/GrVkRenderTarget.h
+++ b/src/gpu/vk/GrVkRenderTarget.h
@@ -126,6 +126,8 @@ private:
                      const GrVkImageView* colorAttachmentView,
                      GrVkImage::Wrapped wrapped);
 
+    constexpr static Flags ComputeFlags(/*???*/);
+
     static GrVkRenderTarget* Create(GrVkGpu*, SkBudgeted, const GrSurfaceDesc&,
                                     const GrVkImageInfo&, GrVkImage::Wrapped wrapped);