Convert use_seccomp_bpf to the BUILDFLAG system.

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 025c857d6dccc28166f873a9d0c1d6e3178dfe5a..aab62b00c25f744edc8f32a25134587cc23305e4 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -15,6 +15,7 @@ import("//media/media_options.gni")
 import("//net/features.gni")
 import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
+import("//sandbox/features.gni")
 import("//third_party/protobuf/proto_library.gni")
 import("//ui/base/ui_features.gni")
 
@@ -2592,6 +2593,7 @@ split_static_library("browser") {
       "//components/cdm/browser",
       "//components/payments/android:payments_jni",
       "//components/resources:components_resources",
+      "//sandbox:sandbox_features",
       "//third_party/android_opengl/etc1",
       "//third_party/android_tools:cpu_features",
       "//third_party/libaddressinput:util",
@@ -2599,7 +2601,6 @@ split_static_library("browser") {
 
     deps -= [ "//components/storage_monitor" ]
     if (use_seccomp_bpf) {
-      defines += [ "USE_SECCOMP_BPF" ]
       deps += [ "//sandbox/linux:seccomp_bpf" ]
     }
   } else {