Revert "Move some Gr headers from include/gpu to include/private and src/gpu."

This reverts commit 3bdc3f5f9c0bf62c1eb7932f67f9f4838782a36a.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Move some Gr headers from include/gpu to include/private and src/gpu.
> 
> Bug: skia:
> Change-Id: I341dd3bff63cc99d3be830e21673073645a9cfec
> Reviewed-on: https://skia-review.googlesource.com/c/176220
> Auto-Submit: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Robert Phillips <robertphillips@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>

TBR=bsalomon@google.com,robertphillips@google.com

Change-Id: Ia7082f66abb969b20dd5d1f12c3c8d09cd84c241
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/176587
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index bfe8dc7..915e06f 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -16,7 +16,12 @@
   "$_include/gpu/GrContextOptions.h",
   "$_include/gpu/GrContext.h",
   "$_include/gpu/GrDriverBugWorkarounds.h",
+  "$_include/gpu/GrGpuResource.h",
+  "$_include/gpu/GrRenderTarget.h",
+  "$_include/gpu/GrResourceKey.h",
+  "$_include/gpu/GrSurface.h",
   "$_include/gpu/GrTexture.h",
+  "$_include/gpu/GrSamplerState.h",
   "$_include/gpu/GrTypes.h",
 
   "$_include/gpu/gl/GrGLAssembleInterface.h",
@@ -31,14 +36,10 @@
   "$_include/private/GrColor.h",
   "$_include/private/GrCCClipPath.h",
   "$_include/private/GrCCPerOpListPaths.h",
-  "$_include/private/GrGpuResource.h",
   "$_include/private/GrOpList.h",
   "$_include/private/GrProxyRef.h",
   "$_include/private/GrSingleOwner.h",
   "$_include/private/GrRenderTargetProxy.h",
-  "$_include/private/GrResourceKey.h",
-  "$_include/private/GrSamplerState.h",
-  "$_include/private/GrSurface.h",
   "$_include/private/GrSurfaceProxy.h",
   "$_include/private/GrTextureProxy.h",
   "$_include/private/GrTypesPriv.h",
@@ -152,7 +153,6 @@
   "$_src/gpu/GrRenderTargetProxyPriv.h",
   "$_src/gpu/GrReducedClip.cpp",
   "$_src/gpu/GrReducedClip.h",
-  "$_src/gpu/GrRenderTarget.h",
   "$_src/gpu/GrRenderTargetContext.cpp",
   "$_src/gpu/GrRenderTargetContext.h",
   "$_src/gpu/GrRenderTargetContextPriv.h",