Instead of optionally compile VP9 source files based on the value of the GN argument 'rtc_libvpx_build_vp9', this CL uses the preprocessor macro RTC_ENABLE_VP9 to decide if VP9 related code needs to be compiled or not. Bug: None Change-Id: I5c1b69d7ec35e8446181d98c912277d0ae8fdba2 Reviewed-on: https://webrtc-review.googlesource.com/c/111063 Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Philip Eliasson <philipel@webrtc.org> Cr-Commit-Position: refs/heads/master@{#25685}