Wires up existing libvpx_build_vp9==0 GYP flag into WebRTC and makes VP9
optional. Change is GYP only for now since libvpx's GN files build VP9
unconditionally.
BUG=webrtc:5884
R=kjellander@webrtc.org
Review URL: https://codereview.webrtc.org/1970343002 .
Cr-Commit-Position: refs/heads/master@{#12741}
