FFmpeg video recording is black with Xvfb and chrome headless on Centos 7

I am trying to do a video recording of headless chrome session on Centos 7 (Amazon EC2 instance) using ffmpeg. I have installed ffmpeg, Xvfb and google chrome on the machine. I started Xvfb on :99, verified the display using xdpyinfo and started chrome. However when I run the ffmpeg cmd to capture the video (no errors in ffmpeg debug logs), the output is dark with a X sign in the centre of video screen. What am I doing wrong here? Any help is appreciated.

**Xvfb and chrome cmds:**
Xvfb :99 -screen 0 1920x1080x24 &
export DISPLAY=:99
google-chrome --headless --disable-gpu --no-sandbox --start-maximized --window-size=1920x1080 https://www.google.com

**FFmpeg cmd:**
ffmpeg -video_size 1920x1080 -framerate 30 -f x11grab -i :99 -loglevel debug -pix_fmt yuv420p /tmp/video.mp4