skia_directwrite -> skia_gdi

This will require lots of rebaselines and a bot CL to swap our lone direct write bot to a lone GDI bot.

BUG=skia:
R=bungeman@google.com

Review URL: https://codereview.chromium.org/394223003
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index c3714dc..dbd0c29 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -139,7 +139,7 @@
       'skia_resource_cache_mb_limit%': 0,
       'skia_resource_cache_count_limit%': 0,
       'skia_angle%': 0,
-      'skia_directwrite%': 0,
+      'skia_gdi%': 0,
       'skia_gpu%': 1,
       'skia_osx_deployment_target%': '',
       'skia_profile_enabled%': 0,