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/RTCDefaultVideoEncoderFactory.m b/sdk/objc/components/video_codec/RTCDefaultVideoEncoderFactory.m
index b01c074..b72296b 100644
--- a/sdk/objc/components/video_codec/RTCDefaultVideoEncoderFactory.m
+++ b/sdk/objc/components/video_codec/RTCDefaultVideoEncoderFactory.m
@@ -15,7 +15,7 @@
 #import "api/video_codec/RTCVideoCodecConstants.h"
 #import "api/video_codec/RTCVideoEncoderVP8.h"
 #import "base/RTCVideoCodecInfo.h"
-#if !defined(RTC_DISABLE_VP9)
+#if defined(RTC_ENABLE_VP9)
 #import "api/video_codec/RTCVideoEncoderVP9.h"
 #endif
 
@@ -44,7 +44,7 @@
 
   RTCVideoCodecInfo *vp8Info = [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp8Name];
 
-#if !defined(RTC_DISABLE_VP9)
+#if defined(RTC_ENABLE_VP9)
   RTCVideoCodecInfo *vp9Info = [[RTCVideoCodecInfo alloc] initWithName:kRTCVideoCodecVp9Name];
 #endif
 
@@ -52,7 +52,7 @@
     constrainedHighInfo,
     constrainedBaselineInfo,
     vp8Info,
-#if !defined(RTC_DISABLE_VP9)
+#if defined(RTC_ENABLE_VP9)
     vp9Info,
 #endif
   ];
@@ -63,7 +63,7 @@
     return [[RTCVideoEncoderH264 alloc] initWithCodecInfo:info];
   } else if ([info.name isEqualToString:kRTCVideoCodecVp8Name]) {
     return [RTCVideoEncoderVP8 vp8Encoder];
-#if !defined(RTC_DISABLE_VP9)
+#if defined(RTC_ENABLE_VP9)
   } else if ([info.name isEqualToString:kRTCVideoCodecVp9Name]) {
     return [RTCVideoEncoderVP9 vp9Encoder];
 #endif