Make SkYUVAIndex publicly accessible

Change-Id: I3da7c566cdf9256e57ac7e7f30fee18b9c1a144d
Reviewed-on: https://skia-review.googlesource.com/c/159460
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/src/image/SkImage_Gpu.h b/src/image/SkImage_Gpu.h
index 87892c4..1f477fd 100644
--- a/src/image/SkImage_Gpu.h
+++ b/src/image/SkImage_Gpu.h
@@ -19,6 +19,7 @@
 class GrTexture;
 
 class SkBitmap;
+struct SkYUVAIndex;
 
 class SkImage_Gpu : public SkImage_Base {
 public: