Introduce ThreadTaskRunnerHandle::OverrideForTesting and TestMockTimeTaskRunner::ScopedContext.

components/sync/driver/sync_stopped_reporter.cc

Index: components/sync/driver/sync_stopped_reporter.cc
diff --git a/components/sync/driver/sync_stopped_reporter.cc b/components/sync/driver/sync_stopped_reporter.cc
index 7daf57c25eba6a07d5b6d0d60b34291a11cafef0..5dd869cb833bacf5f988a4346e27a819da9b18b8 100644
--- a/components/sync/driver/sync_stopped_reporter.cc
+++ b/components/sync/driver/sync_stopped_reporter.cc
@@ -111,9 +111,4 @@ GURL SyncStoppedReporter::GetSyncEventURL(const GURL& sync_service_url) {
   return sync_service_url.ReplaceComponents(replacements);
 }
 
-void SyncStoppedReporter::SetTimerTaskRunnerForTest(
-    const scoped_refptr<base::SingleThreadTaskRunner>& task_runner) {
-  timer_.SetTaskRunner(task_runner);
-}
-
 }  // namespace syncer