ui: Cleanup class/struct forward declarations

components/sync/driver/glue/sync_backend_host_mock.h

Index: components/sync/driver/glue/sync_backend_host_mock.h
diff --git a/components/sync/driver/glue/sync_backend_host_mock.h b/components/sync/driver/glue/sync_backend_host_mock.h
index 2ca3b68ad7ac8566615c74c2369e394ffad460cc..752e562ce9856029942c8e1b9a8a76bc316181b0 100644
--- a/components/sync/driver/glue/sync_backend_host_mock.h
+++ b/components/sync/driver/glue/sync_backend_host_mock.h
@@ -28,7 +28,7 @@ class SyncBackendHostMock : public SyncBackendHost {
 
   void Initialize(
       SyncFrontend* frontend,
-      std::unique_ptr<base::Thread> sync_thread,
+      base::Thread* sync_thread,
       const scoped_refptr<base::SingleThreadTaskRunner>& db_thread,
       const scoped_refptr<base::SingleThreadTaskRunner>& file_thread,
       const WeakHandle<JsEventHandler>& event_handler,
@@ -58,7 +58,7 @@ class SyncBackendHostMock : public SyncBackendHost {
 
   void StopSyncingForShutdown() override;
 
-  std::unique_ptr<base::Thread> Shutdown(ShutdownReason reason) override;
+  void Shutdown(ShutdownReason reason) override;
 
   void UnregisterInvalidationIds() override;
 
@@ -105,8 +105,6 @@ class SyncBackendHostMock : public SyncBackendHost {
   void EnableDirectoryTypeDebugInfoForwarding() override;
   void DisableDirectoryTypeDebugInfoForwarding() override;
 
-  base::MessageLoop* GetSyncLoopForTesting() override;
-
   void RefreshTypesForTest(ModelTypeSet types) override;
 
   void ClearServerData(