Set up a new rtc::Thread instance per test.

Several tests leave pending tasks behind after executing, which may
affect the state of subsequent tests. This CL isolates each test in
the sense that a dedicated Thread instance is created per test and
then pending tasks are flushed and the Thread instance deleted.

Down the line we may want to improve on this and flag those tests
that leave pending tasks/timers etc.

Change-Id: Ibaf3719a9974c57ac2169edca0e2a06a9ea6c78f
Bug: webrtc:11574
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175132
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31268}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index ebd22c6..70caec6 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -407,7 +407,7 @@
   deps = [
     ":video_quality_test_fixture_api",
     "../rtc_base:checks",
-    "../rtc_base:thread_checker",
+    "../rtc_base:platform_thread_types",
   ]
 }