Convert use_seccomp_bpf to the BUILDFLAG system.

content/common/BUILD.gn

Index: content/common/BUILD.gn
diff --git a/content/common/BUILD.gn b/content/common/BUILD.gn
index 4d49b09bfd3eaf54c8c3391500ece920f50d60af..3f3fb75fd0e0dcf46c1f49c00abd6510a249190e 100644
--- a/content/common/BUILD.gn
+++ b/content/common/BUILD.gn
@@ -7,6 +7,7 @@ import("//build/config/ui.gni")
 import("//media/media_options.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//ppapi/features/features.gni")
+import("//sandbox/features.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
 if (is_mac) {
   import("//build/config/mac/mac_sdk.gni")
@@ -390,6 +391,7 @@ source_set("common") {
     "//net",
     "//ppapi/features",
     "//sandbox",
+    "//sandbox:sandbox_features",
     "//services/service_manager",
     "//services/service_manager/public/cpp",
     "//services/service_manager/public/interfaces",
@@ -501,9 +503,7 @@ source_set("common") {
     sources -= [ "cursors/webcursor_aurawin.cc" ]
   }
 
-  if (use_seccomp_bpf) {
-    defines += [ "USE_SECCOMP_BPF" ]
-  } else {
+  if (!use_seccomp_bpf) {
     if (is_linux) {
       sources -= [
         "sandbox_linux/bpf_cros_arm_gpu_policy_linux.cc",