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/sdk/objc/components/video_codec/RTCDefaultVideoDecoderFactory.m b/sdk/objc/components/video_codec/RTCDefaultVideoDecoderFactory.m
index c7d08ca..bdb1851 100644
--- a/sdk/objc/components/video_codec/RTCDefaultVideoDecoderFactory.m
+++ b/sdk/objc/components/video_codec/RTCDefaultVideoDecoderFactory.m
@@ -15,7 +15,7 @@
 #import "api/video_codec/RTCVideoCodecConstants.h"
 #import "api/video_codec/RTCVideoDecoderVP8.h"
 #import "base/RTCVideoCodecInfo.h"
-#if !defined(RTC_DISABLE_VP9)
+#if defined(RTC_ENABLE_VP9)
 #import "api/video_codec/RTCVideoDecoderVP9.h"
 #endif
 
@@ -26,7 +26,7 @@
     return [[RTCVideoDecoderH264 alloc] init];
   } else if ([info.name isEqualToString:kRTCVideoCodecVp8Name]) {
     return [RTCVideoDecoderVP8 vp8Decoder];
-#if !defined(RTC_DISABLE_VP9)
+#if defined(RTC_ENABLE_VP9)
   } else if ([info.name isEqualToString:kRTCVideoCodecVp9Name]) {
     return [RTCVideoDecoderVP9 vp9Decoder];
 #endif
@@ -39,7 +39,7 @@
   return @[
     [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecH264Name],
     [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp8Name],
-#if !defined(RTC_DISABLE_VP9)
+#if defined(RTC_ENABLE_VP9)
     [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp9Name],
 #endif
   ];