Revert "Revert "move GrColor.h to private, IWYU""

This reverts commit 63cc29304a65039feaf27842d587ed16de010b0d.

Reason for revert: pdfium updated

Original change's description:
> Revert "move GrColor.h to private, IWYU"
> 
> This reverts commit e602f395813aab8242afad356008b8e79911adbb.
> 
> Reason for revert: Breaks PDFIUM
> 
> Original change's description:
> > move GrColor.h to private, IWYU
> > 
> > Bug: skia:
> > Change-Id: I0f0dabd7cc54cb7786f53bd6da0c0c012375037e
> > Reviewed-on: https://skia-review.googlesource.com/104160
> > Commit-Queue: Mike Reed <reed@google.com>
> > Reviewed-by: Brian Salomon <bsalomon@google.com>
> 
> TBR=bsalomon@google.com,reed@google.com
> 
> Change-Id: Ifaa50f8771fa1ca8bc152270efdb1fe27f7210f2
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: skia:
> Reviewed-on: https://skia-review.googlesource.com/104440
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Brian Salomon <bsalomon@google.com>

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

Change-Id: I22fa5c0e0628b5c1f3b5f13879c7d1a4528cc93a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/104561
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 4d8f3d8..8e6f76b 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -12,7 +12,6 @@
   "$_include/gpu/GrBackendSurface.h",
   "$_include/gpu/GrBlend.h",
   "$_include/gpu/GrCaps.h",
-  "$_include/gpu/GrColor.h",
   "$_include/gpu/GrConfig.h",
   "$_include/gpu/GrContextOptions.h",
   "$_include/gpu/GrContext.h",
@@ -34,6 +33,7 @@
 
   # Private includes
   "$_include/private/GrAuditTrail.h",
+  "$_include/private/GrColor.h",
   "$_include/private/GrGLSL.h",
   "$_include/private/GrOpList.h",
   "$_include/private/GrSingleOwner.h",