Use ffmpeg_features buildflags

build/linux/unbundle/ffmpeg.gn

Index: build/linux/unbundle/ffmpeg.gn
diff --git a/build/linux/unbundle/ffmpeg.gn b/build/linux/unbundle/ffmpeg.gn
index f4f0e2634997766fc0b9c238836d7cbf56fa5b30..978298a7f291301efd96b4df4c59aa1201d854d4 100644
--- a/build/linux/unbundle/ffmpeg.gn
+++ b/build/linux/unbundle/ffmpeg.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/buildflag_header.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//build/shim_headers.gni")
 
@@ -13,8 +14,9 @@ pkg_config("system_ffmpeg") {
   ]
 }
 
-config("using_system_ffmpeg") {
-  defines = [ "USE_SYSTEM_FFMPEG=1" ]
+buildflag_header("ffmpeg_features") {
+  header = "ffmpeg_features.h"
+  flags = [ "USE_SYSTEM_FFMPEG=true" ]
 }
 
 shim_headers("ffmpeg_shim") {
@@ -28,10 +30,8 @@ shim_headers("ffmpeg_shim") {
 
 source_set("ffmpeg") {
   deps = [
+    ":ffmpeg_features",
     ":ffmpeg_shim",
   ]
-  public_configs = [
-    ":system_ffmpeg",
-    ":using_system_ffmpeg",
-  ]
+  public_configs = [ ":system_ffmpeg" ]
 }