Backport some tests in /net to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/net.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

Edit: Reverted unintended changes to
      http_network_transaction_unittest.cc

R=mattm@chromium.org

Bug: 891670
Change-Id: Id402d28fb6411eab633d6697da771e5f7baab079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786849
Commit-Queue: Matt Mueller <mattm@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694795}

net/socket/udp_socket_perftest.cc

@@ -91,8 +91,8 @@ void UDPSocketPerfTest::WritePacketsToSocket(UDPClientSocket* socket,
 }
 
 void UDPSocketPerfTest::WriteBenchmark(bool use_nonblocking_io) {
-  base::test::TaskEnvironment task_environment(
-      base::test::TaskEnvironment::MainThreadType::IO);
+  base::test::SingleThreadTaskEnvironment task_environment(
+      base::test::SingleThreadTaskEnvironment::MainThreadType::IO);
   const uint16_t kPort = 9999;
 
   // Setup the server to listen.