Add GN build for Linux sandbox.

ui/gfx/BUILD.gn

 # 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.
 
 import("//build/config/ui.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
@@ skipping 421 matching lines @@
   component("gfx_x11") {
     sources = [
       "x/x11_atom_cache.cc",
       "x/x11_atom_cache.h",
       "x/x11_error_tracker.cc",
       "x/x11_error_tracker.h",
       "x/x11_types.cc",
       "x/x11_types.h",
     ]
 
+    defines = [ "GFX_IMPLEMENTATION" ]
     configs += [ "//build/config/linux:x11" ]
 
     deps = [
       ":gfx_export",
       "//base"
     ]
   }
 }