[third_party/ffmpeg] merge some old changes to BUILD.gn

BUILD.gn and other files for ffmpeg have been rebuilt from a recent
version from Chromium. A couple of important changes were lost and
are restored here.

fxr/610499: [build] Explicitly include "novariant" variant
fxr/508076: [build] Always optimize the build for speed

Test: build issue
Change-Id: Id84007ad4279ef5fec50b178143e1c33edb5a8ac
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/ffmpeg/+/746922
Reviewed-by: Nathan Mulcahey <nmulcahey@google.com>
Reviewed-by: Gulfem Savrun Yeniceri <gulfem@google.com>
1 file changed