Use ScopedTaskEnvironment instead of MessageLoop in chrome unit tests.

chrome/browser/process_singleton_posix_unittest.cc

Index: chrome/browser/process_singleton_posix_unittest.cc
diff --git a/chrome/browser/process_singleton_posix_unittest.cc b/chrome/browser/process_singleton_posix_unittest.cc
index 56808a833e96345890a94d6c55a3716636122915..5166ee48476b4ae939388cc09517c51a400128bb 100644
--- a/chrome/browser/process_singleton_posix_unittest.cc
+++ b/chrome/browser/process_singleton_posix_unittest.cc
@@ -27,6 +27,7 @@
 #include "base/single_thread_task_runner.h"
 #include "base/strings/stringprintf.h"
 #include "base/synchronization/waitable_event.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/test/test_timeouts.h"
 #include "base/test/thread_test_helper.h"
 #include "base/threading/thread.h"
@@ -257,7 +258,7 @@ class ProcessSingletonPosixTest : public testing::Test {
     kill_callbacks_++;
   }
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   content::TestBrowserThread io_thread_;
   base::ScopedTempDir temp_dir_;
   base::WaitableEvent wait_event_;