[epiphany] TechPreview: Enable internal WebKit audio mixer

[Marge Bot <marge-bot src gnome org>]

commit 9dfce4fca78a86e0668500272a116613a0a973b1
Author: Philippe Normand <philn igalia com>
Date:   Fri Dec 3 18:10:23 2021 +0000

    TechPreview: Enable internal WebKit audio mixer
    
    Change similar to d5ff4722c but for the Devel Flatpak manifest.
    
    Part-of: <https://gitlab.gnome.org/GNOME/epiphany/-/merge_requests/1039>

 org.gnome.Epiphany.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/org.gnome.Epiphany.json b/org.gnome.Epiphany.json
index 9f37ea823..c46badcac 100644
--- a/org.gnome.Epiphany.json
+++ b/org.gnome.Epiphany.json
@@ -17,7 +17,8 @@
         "--socket=fallback-x11",
         "--socket=pulseaudio",
         "--socket=wayland",
-        "--system-talk-name=org.freedesktop.GeoClue2"
+        "--system-talk-name=org.freedesktop.GeoClue2",
+        "--env=WEBKIT_GST_ENABLE_AUDIO_MIXER=1"
     ],
     "modules" : [
          {