Switch from RTC_DISABLE_VP9 to RTC_ENABLE_VP9.

RTC_ENABLE_VP9 is more natural to deal with then RTC_DISABLE_VP9.
In all the places this macro is used, WebRTC needs to do more things
so it is easier to "do more if RTC_ENABLE_VP9 is defined" than
"do more if RTC_DISABLE_VP9 is not defined".

Bug: None
Change-Id: If992e5c554173e6af3f030f6e0fd21bd82acf9eb
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/c/111242
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25679}
diff --git a/BUILD.gn b/BUILD.gn
index c67602c..5e0503d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -223,8 +223,8 @@
     defines += [ "WEBRTC_INCLUDE_INTERNAL_AUDIO_DEVICE" ]
   }
 
-  if (!rtc_libvpx_build_vp9) {
-    defines += [ "RTC_DISABLE_VP9" ]
+  if (rtc_libvpx_build_vp9) {
+    defines += [ "RTC_ENABLE_VP9" ]
   }
 
   if (rtc_enable_sctp) {