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/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 564989a..896076b 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -1077,8 +1077,10 @@
   deps = [
     ":checks",
     ":macromagic",
+    ":rtc_base_approved",
     ":rtc_event",
     ":rtc_task_queue",
+    "../api/task_queue",
     "../api/task_queue:default_task_queue_factory",
     "task_utils:to_queued_task",
     "//third_party/abseil-cpp/absl/strings",