Enable gn check for //samples:pdfium_test

gn check doesn't understand conditional includes, so `// nogncheck` is
needed.
If `pdf_enable_skia` is false, it will still complain that the headers
files live under `//skia`, which isn't included as in deps.

Change-Id: I2a6e1dd27dccf681d93b01204da15ccbc824b257
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/71751
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/samples/pdfium_test_write_helper.h b/samples/pdfium_test_write_helper.h
index 0de3418..bb4b1ac 100644
--- a/samples/pdfium_test_write_helper.h
+++ b/samples/pdfium_test_write_helper.h
@@ -10,8 +10,8 @@
 #include "public/fpdfview.h"
 
 #ifdef PDF_ENABLE_SKIA
-#include "third_party/skia/include/core/SkPictureRecorder.h"
-#include "third_party/skia/include/core/SkStream.h"
+#include "third_party/skia/include/core/SkPictureRecorder.h"  // nogncheck
+#include "third_party/skia/include/core/SkStream.h"  // nogncheck
 #endif
 
 std::string WritePpm(const char* pdf_name,