Use ScopedTaskEnvironment instead of MessageLoop in components unit tests.

components/sync/base/weak_handle_unittest.cc

Index: components/sync/base/weak_handle_unittest.cc
diff --git a/components/sync/base/weak_handle_unittest.cc b/components/sync/base/weak_handle_unittest.cc
index 103c54eaf1e7b7b1bd02c6987cf7f13a09b31345..b128f1a4b81523c16e59a0fd82842e0d503f618e 100644
--- a/components/sync/base/weak_handle_unittest.cc
+++ b/components/sync/base/weak_handle_unittest.cc
@@ -5,6 +5,7 @@
 #include "components/sync/base/weak_handle.h"
 
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -62,7 +63,7 @@ class WeakHandleTest : public ::testing::Test {
     h.Call(from_here, &Base::Test);
   }
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 };
 
 TEST_F(WeakHandleTest, Uninitialized) {