Updating SocketManager test TaskRunner creation

Recent refactor enforces explicitly tagging a default TaskRunner as
coming from the 'base::ThreadPool()'.

Bug: none
Test: unittests now build
Change-Id: I93956800cceb92c9e302c1047d45eb713d3aae39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776399Reviewed-by: Nikita Podguzov <nikitapodguzov@google.com>
Commit-Queue: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691688}

chrome/services/cups_proxy/socket_manager_unittest.cc

@@ -60,7 +60,7 @@ class FakeServiceDelegate : public FakeCupsProxyServiceDelegate {
   // Note: Can't simulate actual IO thread in unit_tests, so we serve an
   // arbitrary SingleThreadTaskRunner.
   scoped_refptr<base::SingleThreadTaskRunner> GetIOTaskRunner() override {
-    return base::CreateSingleThreadTaskRunner({});
+    return base::CreateSingleThreadTaskRunner({base::ThreadPool()});
   }
 };