Enable VP9 flexible mode if it is not disabled by the kill-switch

Forgot negation in https://webrtc-review.googlesource.com/c/src/+/355700

Bug: webrtc:329396373
Change-Id: I6217b5b5cdb11b182405ae7b3d3d3c1fa5036bc7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/355862
Auto-Submit: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42556}
diff --git a/media/engine/webrtc_video_engine.cc b/media/engine/webrtc_video_engine.cc
index 33ad546..75031fe 100644
--- a/media/engine/webrtc_video_engine.cc
+++ b/media/engine/webrtc_video_engine.cc
@@ -990,7 +990,7 @@
 
       // TODO(webrtc:329396373): Remove after flexible mode is fully deployed.
       vp9_settings.flexibleMode =
-          IsDisabled(call_->trials(), "WebRTC-Video-Vp9FlexibleMode");
+          !IsDisabled(call_->trials(), "WebRTC-Video-Vp9FlexibleMode");
     } else {
       // Multiple spatial layers vp9 screenshare needs flexible mode.
       vp9_settings.flexibleMode = vp9_settings.numberOfSpatialLayers > 1;