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/tests/SpecialImageTest.cpp b/tests/SpecialImageTest.cpp
index f9a0147..d6922c3 100644
--- a/tests/SpecialImageTest.cpp
+++ b/tests/SpecialImageTest.cpp
@@ -18,6 +18,7 @@
 #if SK_SUPPORT_GPU
 #include "GrContext.h"
 #include "GrSurfaceProxy.h"
+#include "GrTextureProxy.h"
 #include "SkGr.h"
 #endif