Mark built-in software video codecs as poisonous.

The goal is to make these injectable, and only VP8 and VP9 specific
targets should depend on them.

Bug: webrtc:7925
Change-Id: Ie9239a54d197fe70c93de0582797211fef6997a2
Reviewed-on: https://webrtc-review.googlesource.com/72082
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23021}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 94ed84e..3e604f0 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -268,7 +268,10 @@
 
 rtc_source_set("video_stream_decoder_create") {
   visibility = [ "*" ]
-  allow_poison = [ "audio_codecs" ]  # TODO(bugs.webrtc.org/8396): Remove.
+  allow_poison = [
+    "audio_codecs",  # TODO(bugs.webrtc.org/8396): Remove.
+    "software_video_codecs",  # TODO(bugs.webrtc.org/7925): Remove.
+  ]
   sources = [
     "video/video_stream_decoder_create.cc",
     "video/video_stream_decoder_create.h",