Make the Skia GN build work.

skia/skia_pdfium.gypi

Index: skia/skia_pdfium.gypi
diff --git a/skia/skia_pdfium.gypi b/skia/skia_pdfium.gypi
index 0de3f84d7218f826bd5a62e290b8acbaa23036aa..5c80a4b9fdcb3c654c8d6113d7dea459b3309eed 100644
--- a/skia/skia_pdfium.gypi
+++ b/skia/skia_pdfium.gypi
@@ -3,7 +3,7 @@
 # found in the LICENSE file.
 
 
-# This gypi file contains all the Chrome-specific enhancements to Skia.
+# This gypi file contains all the PDFium-specific enhancements to Skia.
 # In component mode (shared_lib) it is folded into a single shared library with
 # the Skia files but in all other cases it is a separate library.
 {
@@ -24,6 +24,7 @@
     ],
   },
   'sources': [
+    'config/SkUserConfig.h',
     'ext/google_logging.cc',
   ],
 }