Use ScopedTaskEnvironment instead of MessageLoop in content unit tests.

content/browser/quota/usage_tracker_unittest.cc

Index: content/browser/quota/usage_tracker_unittest.cc
diff --git a/content/browser/quota/usage_tracker_unittest.cc b/content/browser/quota/usage_tracker_unittest.cc
index 98c6831ccd0280d7ee1fdcc8f2357c980b5dcbd7..476d298e806e486efac2a57066c1d753371306c0 100644
--- a/content/browser/quota/usage_tracker_unittest.cc
+++ b/content/browser/quota/usage_tracker_unittest.cc
@@ -9,6 +9,7 @@
 #include "base/macros.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/public/test/mock_special_storage_policy.h"
 #include "net/base/url_util.h"
@@ -205,7 +206,7 @@ class UsageTrackerTest : public testing::Test {
     return client_list;
   }
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   scoped_refptr<MockSpecialStoragePolicy> storage_policy_;
   MockQuotaClient quota_client_;