Revert "make SkClipStack.h private (in src)"

This reverts commit 3efca0a2a8.

Reason for revert: update caller in GraphicsContext.cpp

Original change's description:
> make SkClipStack.h private (in src)
> 
> BUG=skia:
> 
> Change-Id: I05f1140fe483f4a92093cb9783c6e9f067420d30
> Reviewed-on: https://skia-review.googlesource.com/8481
> Reviewed-by: Florin Malita <fmalita@chromium.org>
> Commit-Queue: Mike Reed <reed@google.com>
> 

TBR=fmalita@chromium.org,reed@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Change-Id: Iff1138d72ff2d1187d522487fe76ea293f706f60
Reviewed-on: https://skia-review.googlesource.com/8501
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>

gn/core.gni

@@ -68,7 +68,6 @@ skia_core_sources = [
   "$_src/core/SkCanvasPriv.h",
   "$_src/core/SkChunkAlloc.cpp",
   "$_src/core/SkClipStack.cpp",
-  "$_src/core/SkClipStack.h",
   "$_src/core/SkColor.cpp",
   "$_src/core/SkColorFilter.cpp",
   "$_src/core/SkColorFilterShader.cpp",
@@ -384,6 +383,7 @@ skia_core_sources = [
   "$_include/core/SkBBHFactory.h",
   "$_include/core/SkBitmap.h",
   "$_include/core/SkCanvas.h",
+  "$_include/core/SkClipStack.h",
   "$_include/core/SkColor.h",
   "$_include/core/SkColorFilter.h",
   "$_include/core/SkColorPriv.h",