Add AV1 encoder&decoder wrappers for iOS SDK.

It is now possible to use AV1 encoder and decoder on iOS and test
them in apps like AppRTCMobile.

Bug: None
Change-Id: Ifae221020e5abf3809010676862eecd9ffeec5e3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208400
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33378}

sdk/objc/api/video_codec/RTCVideoDecoderVP9.mm

@@ -20,8 +20,20 @@
 @implementation RTC_OBJC_TYPE (RTCVideoDecoderVP9)
 
 + (id<RTC_OBJC_TYPE(RTCVideoDecoder)>)vp9Decoder {
+  std::unique_ptr<webrtc::VideoDecoder> nativeDecoder(webrtc::VP9Decoder::Create());
+  if (nativeDecoder == nullptr) {
+    return nil;
+  }
   return [[RTC_OBJC_TYPE(RTCWrappedNativeVideoDecoder) alloc]
-      initWithNativeDecoder:std::unique_ptr<webrtc::VideoDecoder>(webrtc::VP9Decoder::Create())];
+      initWithNativeDecoder:std::move(nativeDecoder)];
+}
+
++ (bool)isSupported {
+#if defined(RTC_ENABLE_VP9)
+  return true;
+#else
+  return false;
+#endif
 }
 
 @end