Use base::Thread instead of single-threaded SequencedWorkerPool in TestLauncher.

base/test/launcher/test_launcher.h

Index: base/test/launcher/test_launcher.h
diff --git a/base/test/launcher/test_launcher.h b/base/test/launcher/test_launcher.h
index 38980cfd60ac3355f2aff06acacb8e8abc9bd0f8..3f6bbaa5f9a41fbad221693e6d5cf7207567c349 100644
--- a/base/test/launcher/test_launcher.h
+++ b/base/test/launcher/test_launcher.h
@@ -32,6 +32,7 @@ class CommandLine;
 struct LaunchOptions;
 class SequencedWorkerPoolOwner;
 class TestLauncher;
+class Thread;
 
 // Constants for GTest command-line flags.
 extern const char kGTestFilterFlag[];
@@ -165,6 +166,10 @@ class TestLauncher {
   // Called by the delay timer when no output was made for a while.
   void OnOutputTimeout();
 
+  // Returns the TaskRunner to be used to launch child test processes. This
+  // TaskRunner will have TaskShutdownBehavior::BLOCK_SHUTDOWN semantics.
+  scoped_refptr<TaskRunner> GetTaskRunner();
+
   // Make sure we don't accidentally call the wrong methods e.g. on the worker
   // pool thread. With lots of callbacks used this is non-trivial.
   // Should be the first member so that it's destroyed last: when destroying
@@ -227,8 +232,10 @@ class TestLauncher {
   // Number of jobs to run in parallel.
   size_t parallel_jobs_;
 
-  // Worker pool used to launch processes in parallel.
+  // Worker pool used to launch processes in parallel (|worker_thread_| is used
+  // instead if |parallel_jobs == 1|).
   std::unique_ptr<SequencedWorkerPoolOwner> worker_pool_owner_;
+  std::unique_ptr<Thread> worker_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(TestLauncher);
 };