Bug 1568101 - part2 : use a static pref to control the feature. r=jolin

Use a static pref to control this feature and turn it off by default.

Differential Revision: https://phabricator.services.mozilla.com/D40530

--HG--
extra : moz-landing-system : lando

media/webrtc/signaling/src/media-conduit/VideoConduit.cpp

@@ -12,6 +12,7 @@
 #include "YuvStamper.h"
 #include "mozilla/TemplateLib.h"
 #include "mozilla/media/MediaUtils.h"
+#include "mozilla/StaticPrefs_media.h"
 #include "nsComponentManagerUtils.h"
 #include "nsIPrefBranch.h"
 #include "nsIGfxInfo.h"
@@ -1682,6 +1683,13 @@ std::unique_ptr<webrtc::VideoEncoder> WebrtcVideoConduit::CreateEncoder(
   bool enabled = false;
 #endif
 
+  if (StaticPrefs::media_webrtc_platformencoder()) {
+    encoder.reset(MediaDataDecoderCodec::CreateEncoder(aType));
+    if (encoder) {
+      return encoder;
+    }
+  }
+
   switch (aType) {
     case webrtc::VideoCodecType::kVideoCodecH264:
       // get an external encoder

modules/libpref/init/StaticPrefList.yaml

@@ -5727,6 +5727,11 @@
   value: false
   mirror: always
 
+- name: media.webrtc.platformencoder
+  type: bool
+  value: false
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "mousewheel."
 #---------------------------------------------------------------------------