Switch from build-time to run-time flags for Project Spitzer.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 07b7fdee74d31e5956e9f465c6b12511caaae645..8b01806a61818a64047d0a55afc61bd868de58ae 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -20,8 +20,8 @@ declare_args() {
   # decoding of VP9 and VP8A type content.
   media_use_libvpx = true
 
-  # Neither Android nor iOS use ffmpeg, libvpx.
-  if (is_android || is_ios) {
+  # iOS doesn't use ffmpeg, libvpx.
+  if (is_ios) {
     media_use_ffmpeg = false
     media_use_libvpx = false
   }