Merge SendTask implementation for SingleThreadedTaskQueueForTesting and TaskQueueForTest

That allows to use SingleThreadedTaskQueueForTesting via TaskQueueBase interface
but still have access to test-only SendTask function.

Bug: webrtc:10933
Change-Id: I3cc397e55ea2f1ed9e5d885d6a2ccda412beb826
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/156002
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29480}
diff --git a/test/BUILD.gn b/test/BUILD.gn
index ff13621..00b78d4 100644
--- a/test/BUILD.gn
+++ b/test/BUILD.gn
@@ -631,6 +631,7 @@
     "../rtc_base:checks",
     "../rtc_base:deprecation",
     "../rtc_base:rtc_base_approved",
+    "../rtc_base:task_queue_for_test",
     "../rtc_base/task_utils:to_queued_task",
   ]
 }