Introduce ThreadTaskRunnerHandle::OverrideForTesting and TestMockTimeTaskRunner::ScopedContext.

components/sync/driver/sync_stopped_reporter.h

Index: components/sync/driver/sync_stopped_reporter.h
diff --git a/components/sync/driver/sync_stopped_reporter.h b/components/sync/driver/sync_stopped_reporter.h
index dfb5c0ec7d72f5543dde3f614b0ed9bfaa472ad5..a7194f6de92519b0fe815f630953b49aff6b11ad 100644
--- a/components/sync/driver/sync_stopped_reporter.h
+++ b/components/sync/driver/sync_stopped_reporter.h
@@ -43,10 +43,6 @@ class SyncStoppedReporter : public net::URLFetcherDelegate {
   // net::URLFetcherDelegate implementation.
   void OnURLFetchComplete(const net::URLFetcher* source) override;
 
-  // Override the timer's task runner so it can be triggered in tests.
-  void SetTimerTaskRunnerForTest(
-      const scoped_refptr<base::SingleThreadTaskRunner>& task_runner);
-
  private:
   // Convert the base sync URL into the sync event URL.
   static GURL GetSyncEventURL(const GURL& sync_service_url);