pepper: parse context creation attrib list on the plugin side

ui/gfx/ipc/geometry/gfx_ipc_geometry.gyp

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
      {
       # GN version: //ui/gfx/ipc
       'target_name': 'gfx_ipc_geometry',
       'type': '<(component)',
       'dependencies': [
         '../../../../base/base.gyp:base',
         '../../../../ipc/ipc.gyp:ipc',
         '../../gfx.gyp:gfx',
         '../../gfx.gyp:gfx_geometry',
       ],
-      'defines': [
-        'GFX_IPC_GEOMETRY_IMPLEMENTATION',
-      ],
-      'include_dirs': [
-        '../../..',
-      ],
-      'sources': [
-        'gfx_param_traits.cc',
-        'gfx_param_traits.h',
-        'gfx_param_traits_macros.h',
+      'includes': [
+        'gfx_ipc_geometry.gypi',
       ],
     },
   ],
 }