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/api/BUILD.gn b/api/BUILD.gn
index 01d79ea..645daee 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -38,6 +38,7 @@
 if (!build_with_chromium) {
   rtc_static_library("create_peerconnection_factory") {
     visibility = [ "*" ]
+    allow_poison = [ "default_task_queue" ]
     sources = [
       "create_peerconnection_factory.cc",
       "create_peerconnection_factory.h",