Bug 1676289 - always allow webrtc VP9 encoder to drop frames;r=dminor

Differential Revision: https://phabricator.services.mozilla.com/D96476

dom/media/webrtc/libwebrtcglue/VideoConduit.cpp

@@ -814,7 +814,7 @@ ConfigureVideoEncoderSettings(const VideoCodecConfig* aConfig,
     }
     // VP9 denoising is disabled by default.
     vp9_settings.denoisingOn = codec_default_denoising ? false : denoising;
-    vp9_settings.frameDroppingOn = frame_dropping;
+    vp9_settings.frameDroppingOn = true;  // This must be true for VP9
     return new rtc::RefCountedObject<
         webrtc::VideoEncoderConfig::Vp9EncoderSpecificSettings>(vp9_settings);
   }