ui: Cleanup class/struct forward declarations

components/sync/driver/glue/sync_backend_host.h

Index: components/sync/driver/glue/sync_backend_host.h
diff --git a/components/sync/driver/glue/sync_backend_host.h b/components/sync/driver/glue/sync_backend_host.h
index 987c752e68a06328bbe7c69f22751e181d53b00e..cc6279d0fc73183c3eeab0c4d5b41340a3928d47 100644
--- a/components/sync/driver/glue/sync_backend_host.h
+++ b/components/sync/driver/glue/sync_backend_host.h
@@ -59,7 +59,7 @@ class SyncBackendHost : public BackendDataTypeConfigurer {
   // backend instance. May be null.
   virtual 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,
@@ -119,15 +119,7 @@ class SyncBackendHost : public BackendDataTypeConfigurer {
   // Called on |frontend_loop_| to kick off shutdown.
   // See the implementation and Core::DoShutdown for details.
   // Must be called *after* StopSyncingForShutdown.
-  // For any reason other than BROWSER_SHUTDOWN, caller should claim sync
-  // thread because:
-  // * during browser shutdown sync thread is not claimed to avoid blocking
-  //   browser shutdown on sync shutdown.
-  // * otherwise sync thread is claimed so that if sync backend is recreated
-  //   later, initialization of new backend is serialized on previous sync
-  //   thread after cleanup of previous backend to avoid old/new backends
-  //   interfere with each other.
-  virtual std::unique_ptr<base::Thread> Shutdown(ShutdownReason reason) = 0;
+  virtual void Shutdown(ShutdownReason reason) = 0;
 
   // Removes all current registrations from the backend on the
   // InvalidationService.
@@ -202,8 +194,6 @@ class SyncBackendHost : public BackendDataTypeConfigurer {
   // Disables the sending of directory type debug counters.
   virtual void DisableDirectoryTypeDebugInfoForwarding() = 0;
 
-  virtual base::MessageLoop* GetSyncLoopForTesting() = 0;
-
   // Triggers sync cycle to update |types|.
   virtual void RefreshTypesForTest(ModelTypeSet types) = 0;