Poison default task queue factory

Users of webrtc generally should be able to choose own task queue implementation.
Poison avoids accidental dependency of a low level component on the default implementation
Android and ios apis are still de-facto forced to use the default implementation.

Bug: webrtc:10284
Change-Id: I67ecf2317f43ee32b0c9e8a6e69f1e0987cf1914
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144786
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28524}
diff --git a/BUILD.gn b/BUILD.gn
index 8334573..af0de9f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -675,3 +675,6 @@
 
 group("poison_software_video_codecs") {
 }
+
+group("poison_default_task_queue") {
+}