Add Av1 Decoder wrapper behind a build flag

Bug: webrtc:11404
Change-Id: I090ffd173d667e8845de1b986af462516b7c76e6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169452
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Michael Horowitz <mhoro@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30757}
diff --git a/.gn b/.gn
index 09f1b65..59dd7d6 100644
--- a/.gn
+++ b/.gn
@@ -72,5 +72,7 @@
   # for unittests, it can be disabled (see third_party/libyuv/BUILD.gn)
   libyuv_use_gflags = false
 
+  enable_libaom_decoder = true
+
   gtest_enable_absl_printers = true
 }