ui: Cleanup class/struct forward declarations

components/sync/driver/glue/sync_backend_host_mock.cc

Index: components/sync/driver/glue/sync_backend_host_mock.cc
diff --git a/components/sync/driver/glue/sync_backend_host_mock.cc b/components/sync/driver/glue/sync_backend_host_mock.cc
index 2c351dae638ad9a63b12acc56c65f691ff871795..5857899c0989acf796279c16afe8cd3841c8fe00 100644
--- a/components/sync/driver/glue/sync_backend_host_mock.cc
+++ b/components/sync/driver/glue/sync_backend_host_mock.cc
@@ -16,7 +16,7 @@ SyncBackendHostMock::~SyncBackendHostMock() {}
 
 void SyncBackendHostMock::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,
@@ -53,10 +53,7 @@ bool SyncBackendHostMock::SetDecryptionPassphrase(
 
 void SyncBackendHostMock::StopSyncingForShutdown() {}
 
-std::unique_ptr<base::Thread> SyncBackendHostMock::Shutdown(
-    ShutdownReason reason) {
-  return std::unique_ptr<base::Thread>();
-}
+void SyncBackendHostMock::Shutdown(ShutdownReason reason) {}
 
 void SyncBackendHostMock::UnregisterInvalidationIds() {}
 
@@ -120,10 +117,6 @@ void SyncBackendHostMock::GetModelSafeRoutingInfo(
 
 void SyncBackendHostMock::FlushDirectory() const {}
 
-base::MessageLoop* SyncBackendHostMock::GetSyncLoopForTesting() {
-  return nullptr;
-}
-
 void SyncBackendHostMock::RefreshTypesForTest(ModelTypeSet types) {}
 
 void SyncBackendHostMock::RequestBufferedProtocolEventsAndEnableForwarding() {}