build/linux/unbundle: make ffmpeg.gn define USE_SYSTEM_FFMPEG

This will make it possible for distro-specific patch to be applied
unconditionally.

This does not affect any build at Google. build/linux/unbundle
is a completely parallel directory tree intended to be used
by Linux distribution packagers. Please see
https://cs.chromium.org/chromium/src/build/linux/unbundle/README
for more context.

BUG=none
R=thestig@chromium.org

Review-Url: https://codereview.chromium.org/2811423002 .
Cr-Original-Commit-Position: refs/heads/master@{#464151}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 038501c2703a4439409cd0d7c002b303ffc6377f

linux/unbundle/ffmpeg.gn

@@ -13,6 +13,10 @@ pkg_config("system_ffmpeg") {
   ]
 }
 
+config("using_system_ffmpeg") {
+  defines = [ "USE_SYSTEM_FFMPEG=1" ]
+}
+
 shim_headers("ffmpeg_shim") {
   root_path = "."
   headers = [
@@ -26,5 +30,8 @@ source_set("ffmpeg") {
   deps = [
     ":ffmpeg_shim",
   ]
-  public_configs = [ ":system_ffmpeg" ]
+  public_configs = [
+    ":system_ffmpeg",
+    ":using_system_ffmpeg",
+  ]
 }