Convert use_seccomp_bpf to the BUILDFLAG system.

content/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("//tools/grit/grit_rule.gni")
 import("//build/config/nacl/config.gni")
 import("//ppapi/features/features.gni")
 
 # Applied by targets internal to content.
 config("content_implementation") {
@@ skipping 76 matching lines @@
     set_sources_assignment_filter([])
     sources = [
       "common/sandbox_linux/sandbox_init_linux.cc",
       "common/sandbox_linux/sandbox_seccomp_bpf_linux.cc",
       "common/send_zygote_child_ping_linux.cc",
       "public/common/content_switches.cc",
       "public/common/mojo_channel_switches.cc",
     ]
     set_sources_assignment_filter(sources_assignment_filter)
 
-    defines = [ "USE_SECCOMP_BPF=1" ]
-
     deps = [
       "//base",
       "//media:media_features",
+      "//sandbox:sandbox_features",
       "//sandbox/linux:sandbox",
     ]
   }
 } else {
   group("content") {
     public_deps = content_shared_components
   }
 }
 
 grit("resources") {
@@ skipping 35 matching lines @@
     sources = [
       "app/sandbox_helper_win.cc",
       "public/app/sandbox_helper_win.h",
     ]
 
     deps = [
       "//sandbox",
     ]
   }
 }