make GrPaint.h private -- IWYU

prerequisit: https://codereview.chromium.org/2743533002/

BUG=skia:

Change-Id: I8c2f34a6df165744ddeaeabb562cd6d6a9679b6a
Reviewed-on: https://skia-review.googlesource.com/9461
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 59ba257..7ef9c3a 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -20,7 +20,6 @@
   "$_include/gpu/GrExternalTextureData.h",
   "$_include/gpu/GrFragmentProcessor.h",
   "$_include/gpu/GrGpuResource.h",
-  "$_include/gpu/GrPaint.h",
   "$_include/gpu/GrProcessor.h",
   "$_include/gpu/GrProcessorUnitTest.h",
   "$_include/gpu/GrProgramElement.h",
@@ -118,6 +117,7 @@
   "$_src/gpu/GrOpList.cpp",
   "$_src/gpu/GrOpList.h",
   "$_src/gpu/GrPaint.cpp",
+  "$_src/gpu/GrPaint.h",
   "$_src/gpu/GrPath.cpp",
   "$_src/gpu/GrPath.h",
   "$_src/gpu/GrPathProcessor.cpp",