VP9 frame rate controller is supposed to be used in screen mode only but it was partially enabled in normal video mode. This restricts use of VP9 frame rate controller to screen mode. Bug: chromium:884164 Change-Id: Ie2eaa31f3364a8abccbc4171007708cf7040fc38 Reviewed-on: https://webrtc-review.googlesource.com/100424 Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24769}