Add GrTextureProxyPriv

Several upcoming additions should go in here

Change-Id: I642f3c7cc36b1e6512ee0170640449e88a666d2c
Reviewed-on: https://skia-review.googlesource.com/52661
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index fbc976b..362dac8 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -186,6 +186,7 @@
   "$_src/gpu/GrTextureOpList.cpp",
   "$_src/gpu/GrTextureOpList.h",
   "$_src/gpu/GrTextureProxyCacheAccess.h",
+  "$_src/gpu/GrTextureProxyPriv.h",
   "$_src/gpu/GrTracing.h",
   "$_src/gpu/GrTestUtils.cpp",
   "$_src/gpu/GrTestUtils.h",