Move Skia GN sources to a separate .gni file.

gyp/skia_for_chromium_defines.gypi

 # Copyright 2014 Google Inc.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# THIS FILE IS OBSOLETE. It will be deleted pending a two-sided update to
+# Chromium. Please update the new list in gn/shared_sources.gni.
+
 # This file is included by chrome's skia/skia_common.gypi, and is intended to
 # augment the skia flags that are set there.
 
 {
   'variables': {
 
     # These flags will be defined in chromium
     #
     # If these become 'permanent', they should be moved into skia_common.gypi
     #
     'skia_for_chromium_defines': [
       'SK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS',
     ],
   },
 }