Reexport skia_gdi.

BUG=skia:

Review URL: https://codereview.chromium.org/398043004
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index a5d86e2..e084cd9 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -213,6 +213,7 @@
     'skia_arch_type%': '<(skia_arch_type)',
     'skia_chrome_utils%': '<(skia_chrome_utils)',
     'skia_directwrite%': '<(skia_directwrite)',
+    'skia_gdi%': '<(skia_gdi)',
     'skia_gpu%': '<(skia_gpu)',
     'skia_win_exceptions%': 0,
     'skia_win_ltcg%': 1,