Backport some tests in /ios/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
/ios/net.

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

This CL was uploaded by git cl split.

R=ellyjones@chromium.org

Bug: 891670
Change-Id: I06b784de2f3374bcf57a1b0a39a9df200e67e953
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786969
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693673}

ios/net/protocol_handler_util_unittest.mm

@@ -154,7 +154,7 @@ class ProtocolHandlerUtilTest : public PlatformTest,
   void OnReadCompleted(URLRequest* request, int bytes_read) override {}
 
  protected:
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   URLRequestJobFactoryImpl job_factory_;
   std::unique_ptr<URLRequestContext> request_context_;
 };