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/webrtc.gni b/webrtc.gni
index 9e13283..e27a84a 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -368,6 +368,9 @@
 
   # Software video codecs (VP8 and VP9 through libvpx).
   "software_video_codecs",
+
+  # Default task queue implementation.
+  "default_task_queue",
 ]
 
 absl_include_config = "//third_party/abseil-cpp:absl_include_config"